FlattenMemRefPass crashes on memref.global

`FlattenMemRefPass` crashes deterministically on valid MLIR containing a `memref.global` without a constant dense initializer. The crash occurs both inside `hlstool` and when running the pass directly via `circt-opt`, so the issue is isolated to the pass itself.

Reproducer: [pass_pipeline.mlir](https://github.com/RCoeurjoly/circt-flatten-memref-crash/blob/main/pass_pipeline.mlir). It is too big to attach.

`circt-opt --version`

> LLVM (http://llvm.org/):
> LLVM version 22.0.0git
> Optimized build with assertions.
> CIRCT firtool-1.137.0g20251123_7fbb972
>

`circt-opt pass_pipeline.mlir --flatten-memref -o /dev/null`

> PLEASE submit a bug report to https://github.com/llvm/circt and include the crash backtrace.
> Stack dump:
> 0. Program arguments: /home/roland/circt-nix/result/bin/circt-opt pass_pipeline.mlir --flatten-memref -o /dev/null
> #0 0x000056297990008b llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/home/roland/circt-nix/result/bin/circt-opt+0x11f408b)
> #1 0x00005629798fccba SignalHandler(int, siginfo_t*, void*) (/home/roland/circt-nix/result/bin/circt-opt+0x11f0cba)
> #2 0x00007fd8288769c0 __restore_rt (/nix/store/daamdpmaz2vjvna55ccrc30qw3qb8h6d-glibc-2.40-66/lib/libc.so.6+0x419c0)
> #3 0x000056297b624a4c mlir::DenseElementsAttr::getNumElements() const (/home/roland/circt-nix/result/bin/circt-opt+0x2f18a4c)
> #4 0x000056297b585919 (anonymous namespace)::GlobalOpConversion::matchAndRewrite(mlir::memref::GlobalOp, mlir::memref::GlobalOpAdaptor, mlir::ConversionPatternRewriter&) const (/home/roland/circt-nix/result/bin/circt-opt+0x2e79919)
> #5 0x000056297b57f434 mlir::OpConversionPattern::matchAndRewrite(mlir::Operation*, llvm::ArrayRef, mlir::ConversionPatternRewriter&) const (/home/roland/circt-nix/result/bin/circt-opt+0x2e73434)
> #6 0x000056297c426bfe mlir::ConversionPattern::matchAndRewrite(mlir::Operation*, mlir::PatternRewriter&) const (/home/roland/circt-nix/result/bin/circt-opt+0x3d1abfe)
> #7 0x000056297c47d50c mlir::PatternApplicator::matchAndRewrite(mlir::Operation*, mlir::PatternRewriter&, llvm::function_ref, llvm::function_ref, llvm::function_ref)::'lambda'()::operator()() const (/home/roland/circt-nix/result/bin/circt-opt+0x3d7150c)
> #8 0x000056297c47eaf1 mlir::PatternApplicator::matchAndRewrite(mlir::Operation*, mlir::PatternRewriter&, llvm::function_ref, llvm::function_ref, llvm::function_ref) (/home/roland/circt-nix/result/bin/circt-opt+0x3d72af1)
> #9 0x000056297c42cb5e (anonymous namespace)::OperationLegalizer::legalize(mlir::Operation*) (/home/roland/circt-nix/result/bin/circt-opt+0x3d20b5e)
> #10 0x000056297c42e413 mlir::OperationConverter::convert(mlir::Operation*, bool) (/home/roland/circt-nix/result/bin/circt-opt+0x3d22413)
> #11 0x000056297c42e8aa mlir::OperationConverter::convertOperations(llvm::ArrayRef) (/home/roland/circt-nix/result/bin/circt-opt+0x3d228aa)
> #12 0x000056297c437854 applyConversion(llvm::ArrayRef, mlir::ConversionTarget const&, mlir::FrozenRewritePatternSet const&, mlir::ConversionConfig, (anonymous namespace)::OpConversionMode) (/home/roland/circt-nix/result/bin/circt-opt+0x3d2b854)
> #13 0x000056297c4379ff mlir::applyPartialConversion(mlir::Operation*, mlir::ConversionTarget const&, mlir::FrozenRewritePatternSet const&, mlir::ConversionConfig) (/home/roland/circt-nix/result/bin/circt-opt+0x3d2b9ff)
> #14 0x000056297b584711 (anonymous namespace)::FlattenMemRefPass::runOnOperation() (/home/roland/circt-nix/result/bin/circt-opt+0x2e78711)
> #15 0x000056297c5c3948 mlir::detail::OpToOpPassAdaptor::run(mlir::Pass*, mlir::Operation*, mlir::AnalysisManager, bool, unsigned int) (/home/roland/circt-nix/result/bin/circt-opt+0x3eb7948)
> #16 0x000056297c5c3efb mlir::detail::OpToOpPassAdaptor::runPipeline(mlir::OpPassManager&, mlir::Operation*, mlir::AnalysisManager, bool, unsigned int, mlir::PassInstrumentor*, mlir::PassInstrumentation::PipelineParentInfo const*) (/home/roland/circt-nix/result/bin/circt-opt+0x3eb7efb)
> #17 0x000056297c5c46bd mlir::PassManager::runPasses(mlir::Operation*, mlir::AnalysisManager) (/home/roland/circt-nix/result/bin/circt-opt+0x3eb86bd)
> #18 0x000056297c5c5ab5 mlir::PassManager::run(mlir::Operation*) (/home/roland/circt-nix/result/bin/circt-opt+0x3eb9ab5)
> #19 0x000056297bc8a3cd performActions(llvm::raw_ostream&, std::shared_ptr const&, mlir::MLIRContext*, mlir::MlirOptMainConfig const&) (/home/roland/circt-nix/result/bin/circt-opt+0x357e3cd)
> #20 0x000056297bc8b42c llvm::LogicalResult llvm::function_ref >, llvm::MemoryBufferRef const&, llvm::raw_ostream&)>::callback_fn >, mlir::DialectRegistry&, mlir::MlirOptMainConfig const&)::'lambda'(std::unique_ptr >, llvm::MemoryBufferRef, llvm::raw_ostream&)>(long, std::unique_ptr >, llvm::MemoryBufferRef const&, llvm::raw_ostream&) (/home/roland/circt-nix/result/bin/circt-opt+0x357f42c)
> #21 0x000056297bcc4585 mlir::splitAndProcessBuffer(std::unique_ptr >, llvm::function_ref >, llvm::MemoryBufferRef const&, llvm::raw_ostream&)>, llvm::raw_ostream&, llvm::StringRef, llvm::StringRef) (/home/roland/circt-nix/result/bin/circt-opt+0x35b8585)
> #22 0x000056297bc82891 mlir::MlirOptMain(llvm::raw_ostream&, std::unique_ptr >, mlir::DialectRegistry&, mlir::MlirOptMainConfig const&) (.part.0) (/home/roland/circt-nix/result/bin/circt-opt+0x3576891)
> #23 0x000056297bc8bb04 mlir::MlirOptMain(int, char**, llvm::StringRef, llvm::StringRef, mlir::DialectRegistry&) (/home/roland/circt-nix/result/bin/circt-opt+0x357fb04)
> #24 0x000056297bc8bd55 mlir::MlirOptMain(int, char**, llvm::StringRef, mlir::DialectRegistry&) (/home/roland/circt-nix/result/bin/circt-opt+0x357fd55)
> #25 0x0000562979852c1c main (/home/roland/circt-nix/result/bin/circt-opt+0x1146c1c)
> #26 0x00007fd82885f4d8 __libc_start_call_main (/nix/store/daamdpmaz2vjvna55ccrc30qw3qb8h6d-glibc-2.40-66/lib/libc.so.6+0x2a4d8)
> #27 0x00007fd82885f59b __libc_start_main@GLIBC_2.2.5 (/nix/store/daamdpmaz2vjvna55ccrc30qw3qb8h6d-glibc-2.40-66/lib/libc.so.6+0x2a59b)
> #28 0x0000562979857aa5 _start (/home/roland/circt-nix/result/bin/circt-opt+0x114baa5)
> Segmentation fault (core dumped)
>

Start by running circt-opt with --flatten-memref on the linked pass_pipeline.mlir reproducer. Then inspect FlattenMemRefPass and its GlobalOpConversion, which the stack trace identifies as the crash site. Done means the pass no longer crashes on this valid memref.global input when run through circt-opt or hlstool.

Written by the indexing model from the issue text.