[MLIR] Fix GCC's `-Wreturn-type` warnings (#177654)
This patch fixes `-Wreturn-type` warnings which happens if MLIR is built
with GCC compiler (11.5 is used for detecting)
Founded errors
```
build/llvm-llvmorg-21.1.8/mlir/lib/CAPI/Transforms/Rewrite.cpp: In function ‘MlirGreedyRewriteStrictness mlirGreedyRewriteDriverConfigGetStrictness(MlirGreedyRewriteDriverConfig)’:
build/llvm-llvmorg-21.1.8/mlir/lib/CAPI/Transforms/Rewrite.cpp:399:1: warning: control reaches end of non-void function [-Wreturn-type]
399 | }
| ^
build/llvm-llvmorg-21.1.8/mlir/lib/CAPI/Transforms/Rewrite.cpp: In function ‘MlirGreedySimplifyRegionLevel mlirGreedyRewriteDriverConfigGetRegionSimplificationLevel(MlirGreedyRewriteDriverConfig)’:
build/llvm-llvmorg-21.1.8/mlir/lib/CAPI/Transforms/Rewrite.cpp:414:1: warning: control reaches end of non-void function [-Wreturn-type]
414 | }
| ^
build/llvm-llvmorg-21.1.8/mlir/lib/Dialect/GPU/IR/GPUDialect.cpp: In member function ‘mlir::Speculation::Speculatability mlir::gpu::SubgroupBroadcastOp::getSpeculatability()’:
build/llvm-llvmorg-21.1.8/mlir/lib/Dialect/GPU/IR/GPUDialect.cpp:2522:1: warning: control reaches end of non-void function [-Wreturn-type]
2522 | }
| ^
build/llvm-llvmorg-21.1.8/mlir/lib/Dialect/GPU/IR/GPUDialect.cpp: In member function ‘llvm::LogicalResult mlir::gpu::SubgroupBroadcastOp::verify()’:
build/llvm-llvmorg-21.1.8/mlir/lib/Dialect/GPU/IR/GPUDialect.cpp:2537:1: warning: control reaches end of non-void function [-Wreturn-type]
2537 | }
| ^
build/llvm-llvmorg-21.1.8/mlir/lib/Dialect/ArmNeon/Transforms/LowerContractToNeonPatterns.cpp: In member function ‘mlir::Value {anonymous}::VectorContractRewriter::createMMLA(mlir::PatternRewriter&, mlir::Location, mlir::Value, mlir::Value, mlir::Value)’:
build/llvm-llvmorg-21.1.8/mlir/lib/Dialect/ArmNeon/Transforms/LowerContractToNeonPatterns.cpp:153:3: warning: control reaches end of non-void function [-Wreturn-type]
153 | }
| ^
build/llvm-llvmorg-21.1.8/mlir/lib/Dialect/Linalg/IR/LinalgOps.cpp: In function ‘std::pair<long int, long int> mlir::linalg::getFmrFromWinogradConv2DFmr(mlir::linalg::WinogradConv2DFmr)’:
build/llvm-llvmorg-21.1.8/mlir/lib/Dialect/Linalg/IR/LinalgOps.cpp:3776:1: warning: control reaches end of non-void function [-Wreturn-type]
3776 | }
| ^
build/llvm-llvmorg-21.1.8/mlir/test/lib/Dialect/Test/TestOpDefs.cpp: In function ‘llvm::StringLiteral getVisibilityString(mlir::SymbolTable::Visibility)’:
build/llvm-llvmorg-21.1.8/mlir/test/lib/Dialect/Test/TestOpDefs.cpp:37:1: warning: control reaches end of non-void function [-Wreturn-type]
37 | }
| ^
```
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