[mlir] Fix a warning

This patch fixes:

  llvm-project/mlir/lib/Dialect/Linalg/Transforms/SplitReduction.cpp:300:26:
  error: comparison of integers of different signs: 'int64_t' (aka
  'long') and 'size_t' (aka 'unsigned long') [-Werror,-Wsign-compare]

Author: kazutakahirata

mlir/lib/Dialect/Linalg/Transforms/SplitReduction.cpp

@@ -297,7 +297,7 @@ FailureOr<SplitReductionResult> mlir::linalg::splitReductionByScaling(
     return b.notifyMatchFailure(op, "unknown reduction neutral");
 
   // TODO: relax this when multi-reduction support is available.
-  if (op.getNumOutputs() != neutralElements.size())
+  if (op.getNumOutputs() != (int)neutralElements.size())
     return b.notifyMatchFailure(op, "expect one reduction per output");
 
   // Rewrite part.