[mlir] Assertion `op.getRegion().hasOneBlock() && "expected one block"' failed.

git version: 5492d06d4fb99795b3b7247be374cdcb47f5ac03

system: `Ubuntu 18.04.6 LTS`

reproduce with: `mlir-opt -acc-specialize-for-host=enable-host-fallback a.mlir`

a.mlir:

```mlir
module {
func.func @test(%arg0: memref<10xf64>, %arg1: index) -> f64 {
%0 = acc.create varPtr(%arg0 : memref<10xf64>) -> memref<10xf64>
acc.serial dataOperands(%0 : memref<10xf64>) {
}
%c0 = arith.constant 0 : index
%c10 = arith.constant 10 : index
%1 = affine.load %arg0[%arg1] : memref<10xf64>
affine.for %arg2 = 0 to 10 {
affine.for %arg3 = 0 to 10 {
affine.for %arg4 = 0 to 10 {
%3 = affine.load %0[%arg2] : memref<10xf64>
affine.store %3, %arg0[%arg2] : memref<10xf64>
}
}
}
%2 = affine.load %0[%c0] : memref<10xf64>
return %2 : f64
}
}

```

stack trace:

```
mlir-opt: /data/szy/MLIR/llvm-release/llvm-project/mlir/include/mlir/Dialect/OpenACC/Transforms/ACCSpecializePatterns.h:98: virtual LogicalResult mlir::acc::ACCRegionUnwrapConversion::matchAndRewrite(OpTy, PatternRewriter &) const [OpTy = mlir::acc::SerialOp]: Assertion `op.getRegion().hasOneBlock() && "expected one block"' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace and instructions to reproduce the bug.
Stack dump:
0. Program arguments: /data/szy/MLIR/llvm-release/llvm-project/build/bin/mlir-opt -acc-specialize-for-host=enable-host-fallback 66.mlir
#0 0x000056061ce590f8 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/data/szy/MLIR/llvm-release/llvm-project/build/bin/mlir-opt+0x20300f8)
#1 0x000056061ce55f91 llvm::sys::RunSignalHandlers() (/data/szy/MLIR/llvm-release/llvm-project/build/bin/mlir-opt+0x202cf91)
#2 0x000056061ce5a216 SignalHandler(int, siginfo_t*, void*) Signals.cpp:0:0
#3 0x00007f3c46916420 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x14420)
#4 0x00007f3c45f5300b raise /build/glibc-LcI20x/glibc-2.31/signal/../sysdeps/unix/sysv/linux/raise.c:51:1
#5 0x00007f3c45f32859 abort /build/glibc-LcI20x/glibc-2.31/stdlib/abort.c:81:7
#6 0x00007f3c45f32729 get_sysdep_segment_value /build/glibc-LcI20x/glibc-2.31/intl/loadmsgcat.c:509:8
#7 0x00007f3c45f32729 _nl_load_domain /build/glibc-LcI20x/glibc-2.31/intl/loadmsgcat.c:970:34
#8 0x00007f3c45f43fd6 (/lib/x86_64-linux-gnu/libc.so.6+0x33fd6)
#9 0x000056061d872932 mlir::acc::ACCRegionUnwrapConversion::matchAndRewrite(mlir::acc::SerialOp, mlir::PatternRewriter&) const (/data/szy/MLIR/llvm-release/llvm-project/build/bin/mlir-opt+0x2a49932)
#10 0x00005606257a8b5d void llvm::function_ref::callback_fn, llvm::function_ref, llvm::function_ref)::$_0>(long) PatternApplicator.cpp:0:0
#11 0x00005606257a54ba mlir::PatternApplicator::matchAndRewrite(mlir::Operation*, mlir::PatternRewriter&, llvm::function_ref, llvm::function_ref, llvm::function_ref) (/data/szy/MLIR/llvm-release/llvm-project/build/bin/mlir-opt+0xa97c4ba)
#12 0x000056062577dd94 (anonymous namespace)::GreedyPatternRewriteDriver::processWorklist() GreedyPatternRewriteDriver.cpp:0:0
#13 0x000056062577ce12 void llvm::function_ref::callback_fn<(anonymous namespace)::RegionPatternRewriteDriver::simplify(bool*) &&::$_2>(long) GreedyPatternRewriteDriver.cpp:0:0
#14 0x0000560625779dfb mlir::applyPatternsGreedily(mlir::Region&, mlir::FrozenRewritePatternSet const&, mlir::GreedyRewriteConfig, bool*) (/data/szy/MLIR/llvm-release/llvm-project/build/bin/mlir-opt+0xa950dfb)
#15 0x000056061d876c15 (anonymous namespace)::ACCSpecializeForHost::runOnOperation() ACCSpecializeForHost.cpp:0:0
#16 0x0000560625804502 mlir::detail::OpToOpPassAdaptor::run(mlir::Pass*, mlir::Operation*, mlir::AnalysisManager, bool, unsigned int) (/data/szy/MLIR/llvm-release/llvm-project/build/bin/mlir-opt+0xa9db502)
#17 0x0000560625805344 mlir::detail::OpToOpPassAdaptor::runPipeline(mlir::OpPassManager&, mlir::Operation*, mlir::AnalysisManager, bool, unsigned int, mlir::PassInstrumentor*, mlir::PassInstrumentation::PipelineParentInfo const*) (/data/szy/MLIR/llvm-release/llvm-project/build/bin/mlir-opt+0xa9dc344)
#18 0x000056062580fece auto void mlir::parallelForEach<__gnu_cxx::__normal_iterator>>, mlir::detail::OpToOpPassAdaptor::runOnOperationAsyncImpl(bool)::$_12>(mlir::MLIRContext*, __gnu_cxx::__normal_iterator>>, __gnu_cxx::__normal_iterator>>, mlir::detail::OpToOpPassAdaptor::runOnOperationAsyncImpl(bool)::$_12&&)::'lambda'(__gnu_cxx::__normal_iterator>>&&)::operator()(__gnu_cxx::__normal_iterator>>&&) const Pass.cpp:0:0
#19 0x0000560625807ff8 mlir::detail::OpToOpPassAdaptor::runOnOperationAsyncImpl(bool) (/data/szy/MLIR/llvm-release/llvm-project/build/bin/mlir-opt+0xa9deff8)
#20 0x00005606258046e7 mlir::detail::OpToOpPassAdaptor::run(mlir::Pass*, mlir::Operation*, mlir::AnalysisManager, bool, unsigned int) (/data/szy/MLIR/llvm-release/llvm-project/build/bin/mlir-opt+0xa9db6e7)
#21 0x0000560625805344 mlir::detail::OpToOpPassAdaptor::runPipeline(mlir::OpPassManager&, mlir::Operation*, mlir::AnalysisManager, bool, unsigned int, mlir::PassInstrumentor*, mlir::PassInstrumentation::PipelineParentInfo const*) (/data/szy/MLIR/llvm-release/llvm-project/build/bin/mlir-opt+0xa9dc344)
#22 0x000056062580c25a mlir::PassManager::runPasses(mlir::Operation*, mlir::AnalysisManager) (/data/szy/MLIR/llvm-release/llvm-project/build/bin/mlir-opt+0xa9e325a)
#23 0x000056062580bb8f mlir::PassManager::run(mlir::Operation*) (/data/szy/MLIR/llvm-release/llvm-project/build/bin/mlir-opt+0xa9e2b8f)
#24 0x000056061cf083d0 performActions(llvm::raw_ostream&, std::shared_ptr const&, mlir::MLIRContext*, mlir::MlirOptMainConfig const&) MlirOptMain.cpp:0:0
#25 0x000056061cf07606 llvm::LogicalResult llvm::function_ref>, llvm::MemoryBufferRef const&, llvm::raw_ostream&)>::callback_fn>, mlir::DialectRegistry&, mlir::MlirOptMainConfig const&)::$_0>(long, std::unique_ptr>, llvm::MemoryBufferRef const&, llvm::raw_ostream&) MlirOptMain.cpp:0:0
#26 0x0000560625b588a5 mlir::splitAndProcessBuffer(std::unique_ptr>, llvm::function_ref>, llvm::MemoryBufferRef const&, llvm::raw_ostream&)>, llvm::raw_ostream&, llvm::StringRef, llvm::StringRef) (/data/szy/MLIR/llvm-release/llvm-project/build/bin/mlir-opt+0xad2f8a5)
#27 0x000056061cefe666 mlir::MlirOptMain(llvm::raw_ostream&, std::unique_ptr>, mlir::DialectRegistry&, mlir::MlirOptMainConfig const&) (/data/szy/MLIR/llvm-release/llvm-project/build/bin/mlir-opt+0x20d5666)
#28 0x000056061cefea45 mlir::MlirOptMain(int, char**, llvm::StringRef, llvm::StringRef, mlir::DialectRegistry&) (/data/szy/MLIR/llvm-release/llvm-project/build/bin/mlir-opt+0x20d5a45)
#29 0x000056061cefeda5 mlir::MlirOptMain(int, char**, llvm::StringRef, mlir::DialectRegistry&) (/data/szy/MLIR/llvm-release/llvm-project/build/bin/mlir-opt+0x20d5da5)
#30 0x000056061ce3eb82 main (/data/szy/MLIR/llvm-release/llvm-project/build/bin/mlir-opt+0x2015b82)
#31 0x00007f3c45f34083 __libc_start_main /build/glibc-LcI20x/glibc-2.31/csu/../csu/libc-start.c:342:3
#32 0x000056061ce3e6be _start (/data/szy/MLIR/llvm-release/llvm-project/build/bin/mlir-opt+0x20156be)
Aborted (core dumped)
```

Reproduce the crash with mlir-opt -acc-specialize-for-host=enable-host-fallback using the a.mlir input. Start at ACCRegionUnwrapConversion in ACCSpecializePatterns.h:98 and the ACCSpecializeForHost pass in ACCSpecializeForHost.cpp; done means the reproducer no longer aborts on the one-block assertion.

Written by the indexing model from the issue text.