[CFToHandshake][DialectConversion] When rollback is removed, PartialLowerRegion becomes illegal.

See the discussion here: https://discourse.llvm.org/t/rfc-a-new-one-shot-dialect-conversion-driver/79083/46

I followed the steps to set the flag in MLIR to disallow rollback, and CFToHandshake fails.

```
/scratch/verif/mikeu/circt/build/bin/circt-opt -lower-cf-to-handshake /scratch/verif/mikeu/circt/test/Conversion/CFToHandshake/invalid.mlir -split-input-file -verify-diagnostics # RUN: at line 1
+
LLVM ERROR: pattern 'PartialLowerRegion' rollback of IR modifications requested
PLEASE submit a bug report to https://github.com/llvm/circt and include the crash backtrace.
Stack dump:
0. Program arguments: /scratch/verif/mikeu/circt/build/bin/circt-opt -lower-cf-to-handshake /scratch/verif/mikeu/circt/test/Conversion/CFToHandshake/invalid.mlir -split-input-file -verify-diagnostics
#0 0x0000000001efe17d llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) /scratch/verif/mikeu/circt/llvm/llvm/lib/Support/Unix/Signals.inc:804:11
#1 0x0000000001efe6fb PrintStackTraceSignalHandler(void*) /scratch/verif/mikeu/circt/llvm/llvm/lib/Support/Unix/Signals.inc:888:1
#2 0x0000000001efc696 llvm::sys::RunSignalHandlers() /scratch/verif/mikeu/circt/llvm/llvm/lib/Support/Signals.cpp:105:5
#3 0x0000000001efef3d SignalHandler(int, siginfo_t*, void*) /scratch/verif/mikeu/circt/llvm/llvm/lib/Support/Unix/Signals.inc:418:7
#4 0x00007f6efe168cf0 __restore_rt (/lib64/libpthread.so.0+0x12cf0)
#5 0x00007f6efcb66acf raise (/lib64/libc.so.6+0x4eacf)
#6 0x00007f6efcb39ea5 abort (/lib64/libc.so.6+0x21ea5)
#7 0x0000000001e6a6e4 llvm::report_fatal_error(llvm::Twine const&, bool) /scratch/verif/mikeu/circt/llvm/llvm/lib/Support/ErrorHandling.cpp:126:5
#8 0x00000000063a35e5 mlir::detail::ConversionPatternRewriterImpl::undoRewrites(unsigned int, llvm::StringRef) /scratch/verif/mikeu/circt/llvm/mlir/lib/Transforms/Utils/DialectConversion.cpp:1233:5
#9 0x00000000063b6241 mlir::detail::ConversionPatternRewriterImpl::resetState((anonymous namespace)::RewriterState, llvm::StringRef) /scratch/verif/mikeu/circt/llvm/mlir/lib/Transforms/Utils/DialectConversion.cpp:0:3
#10 0x00000000063b61cb (anonymous namespace)::OperationLegalizer::legalizeWithPattern(mlir::Operation*, mlir::ConversionPatternRewriter&)::$_21::operator()(mlir::Pattern const&) const /scratch/verif/mikeu/circt/llvm/mlir/lib/Transforms/Utils/DialectConversion.cpp:2174:18
#11 0x00000000063b5ecd void llvm::function_ref::callback_fn<(anonymous namespace)::OperationLegalizer::legalizeWithPattern(mlir::Operation*, mlir::ConversionPatternRewriter&)::$_21>(long, mlir::Pattern const&) /scratch/verif/mikeu/circt/llvm/\
llvm/include/llvm/ADT/STLFunctionalExtras.h:46:5
#12 0x000000000644a769 llvm::function_ref::operator()(mlir::Pattern const&) const /scratch/verif/mikeu/circt/llvm/llvm/include/llvm/ADT/STLFunctionalExtras.h:69:5
#13 0x0000000006449577 mlir::PatternApplicator::matchAndRewrite(mlir::Operation*, mlir::PatternRewriter&, llvm::function_ref, llvm::function_ref, llvm::function_ref)::$_\
1::operator()() const /scratch/verif/mikeu/circt/llvm/mlir/lib/Rewrite/PatternApplicator.cpp:231:9
#14 0x00000000064491b5 void llvm::function_ref::callback_fn, llvm::function_ref, llvm::function_ref)::$_1>(long) /scratch/verif/mikeu/circt/llvm/llvm/include/llvm/ADT/STLFunctionalExtras.h:46:5
#15 0x0000000001e3f569 llvm::function_ref::operator()() const /scratch/verif/mikeu/circt/llvm/llvm/include/llvm/ADT/STLFunctionalExtras.h:69:5
#16 0x000000000644ac7d void mlir::MLIRContext::executeAction(llvm::function_ref, llvm::ArrayRef, mlir::Pattern const&) /scratch/verif/mikeu/circt/llvm/mlir/include/mlir/IR/MLIRContext.h:281:3
#17 0x0000000006447b2c mlir::PatternApplicator::matchAndRewrite(mlir::Operation*, mlir::PatternRewriter&, llvm::function_ref, llvm::function_ref, llvm::function_ref) /sc\
ratch/verif/mikeu/circt/llvm/mlir/lib/Rewrite/PatternApplicator.cpp:233:9
#18 0x00000000063b57d4 (anonymous namespace)::OperationLegalizer::legalizeWithPattern(mlir::Operation*, mlir::ConversionPatternRewriter&) /scratch/verif/mikeu/circt/llvm/mlir/lib/Transforms/Utils/DialectConversion.cpp:2197:21
#19 0x00000000063a9b10 (anonymous namespace)::OperationLegalizer::legalize(mlir::Operation*, mlir::ConversionPatternRewriter&) /scratch/verif/mikeu/circt/llvm/mlir/lib/Transforms/Utils/DialectConversion.cpp:2086:17
#20 0x00000000063a9483 mlir::OperationConverter::convert(mlir::ConversionPatternRewriter&, mlir::Operation*) /scratch/verif/mikeu/circt/llvm/mlir/lib/Transforms/Utils/DialectConversion.cpp:2596:26
#21 0x00000000063a9e2f mlir::OperationConverter::convertOperations(llvm::ArrayRef) /scratch/verif/mikeu/circt/llvm/mlir/lib/Transforms/Utils/DialectConversion.cpp:2696:16
#22 0x00000000063adb29 mlir::applyPartialConversion(llvm::ArrayRef, mlir::ConversionTarget const&, mlir::FrozenRewritePatternSet const&, mlir::ConversionConfig) /scratch/verif/mikeu/circt/llvm/mlir/lib/Transforms/Utils/DialectConversion.cpp:3388:22
#23 0x00000000063adc2d mlir::applyPartialConversion(mlir::Operation*, mlir::ConversionTarget const&, mlir::FrozenRewritePatternSet const&, mlir::ConversionConfig) /scratch/verif/mikeu/circt/llvm/mlir/lib/Transforms/Utils/DialectConversion.cpp:3394:10
#24 0x00000000047360f9 circt::handshake::partiallyLowerRegion(llvm::function_ref const&, mlir::MLIRContext*, mlir::Region&) /scratch/verif/mikeu/circt/lib/Conversion/CFToHandshake/CFToHandshake.cpp:20\
6:7
#25 0x0000000003592a49 llvm::LogicalResult circt::handshake::runPartialLowering>, llvm::DenseMap, llvm::detail::DenseMapPair>, llvm::SmallVector>>, 0u>>, bool, llvm::MapVector>, llvm::DenseMap, llvm::detail::DenseMapPair>, llvm::SmallVector>>, 0u>>, bo\
ol>(circt::handshake::HandshakeLowering&, llvm::LogicalResult (circt::handshake::HandshakeLowering::*)(mlir::ConversionPatternRewriter&, llvm::MapVector>, llvm::DenseMap, llvm::detail::DenseMapPair>, llvm::SmallVector>>, 0u>>, bool), llvm::MapVector>, llvm::DenseMap, llvm::detail::DenseMapPair>, llvm::SmallVector>>, 0u>>&, bool&) /scratch/verif/mikeu/circt/include/circt/Conversion/CFToHandshake.h:175:10
#26 0x0000000004759b43 llvm::LogicalResult circt::handshake::lowerRegion(circt::handshake::HandshakeLowering&, bool, bool, mlir::Value) /scratch/verif/mikeu/circt/include/circt/Conversion/CFToHandshake.h:230:14
#27 0x0000000004740377 lowerFuncOp(mlir::func::FuncOp, mlir::MLIRContext*, bool, bool) /scratch/verif/mikeu/circt/lib/Conversion/CFToHandshake/CFToHandshake.cpp:1732:16
#28 0x000000000473fc84 (anonymous namespace)::CFToHandshakePass::runOnOperation() /scratch/verif/mikeu/circt/lib/Conversion/CFToHandshake/CFToHandshake.cpp:1757:18
#29 0x00000000066fabeb mlir::detail::OpToOpPassAdaptor::run(mlir::Pass*, mlir::Operation*, mlir::AnalysisManager, bool, unsigned int)::$_7::operator()() const /scratch/verif/mikeu/circt/llvm/mlir/lib/Pass/Pass.cpp:0:17
#30 0x00000000066fab85 void llvm::function_ref::callback_fn(long) /scratch/verif/mikeu/circt/llvm/llvm/include/llvm/ADT/STLFunctionalExtras.h:46:\
5
#31 0x0000000001e3f569 llvm::function_ref::operator()() const /scratch/verif/mikeu/circt/llvm/llvm/include/llvm/ADT/STLFunctionalExtras.h:69:5
#32 0x00000000066feb9d void mlir::MLIRContext::executeAction(llvm::function_ref, llvm::ArrayRef, mlir::Pass&) /scratch/verif/mikeu/circt/llvm/mlir/include/mlir/IR/MLIRContext.h:281:3
#33 0x00000000066f62aa mlir::detail::OpToOpPassAdaptor::run(mlir::Pass*, mlir::Operation*, mlir::AnalysisManager, bool, unsigned int) /scratch/verif/mikeu/circt/llvm/mlir/lib/Pass/Pass.cpp:534:17
#34 0x00000000066f6864 mlir::detail::OpToOpPassAdaptor::runPipeline(mlir::OpPassManager&, mlir::Operation*, mlir::AnalysisManager, bool, unsigned int, mlir::PassInstrumentor*, mlir::PassInstrumentation::PipelineParentInfo const*) /scratch/verif/mikeu/circt/llvm/mlir/lib/\
Pass/Pass.cpp:594:16
#35 0x00000000066f834c mlir::PassManager::runPasses(mlir::Operation*, mlir::AnalysisManager) /scratch/verif/mikeu/circt/llvm/mlir/lib/Pass/Pass.cpp:907:10
#36 0x00000000066f826f mlir::PassManager::run(mlir::Operation*) /scratch/verif/mikeu/circt/llvm/mlir/lib/Pass/Pass.cpp:887:60
...
```

@mortbopet do we need to maintain this PartialLowerRegion? At the very least, I don't think it needs to be a DialectConversion, since it just does a modifyOpInPlace and calls some function.

Seems like specifically the issue is `maximizeSSA`, which is making some IR modifications before potentially returning failure. If we _really_ care about this being a `DialectConversion`, I think it could be made to delay modifications until we are sure the pattern can succeed, but that would require splitting `addArgToTerminator` and building an in-memory IRMapping or other table to keep track of rewrites until we can actually apply them all at the end.

Reproduce the failure with circt-opt on test/Conversion/CFToHandshake/invalid.mlir and the CF-to-Handshake lowering command. Read lib/Conversion/CFToHandshake/CFToHandshake.cpp around partiallyLowerRegion and lowerFuncOp, plus include/circt/Conversion/CFToHandshake.h around runPartialLowering and maximizeSSA. The work is complete when conversion no longer aborts when rollback is disallowed and the test passes.

Written by the indexing model from the issue text.