[MLIR] [NFC] fix unused var warning

Summary:
Fix this warning:
`
[69/106] Building CXX object tools/mlir/lib/Dialect/StandardOps/CMakeFiles/MLIRStandardOps.dir/Ops.cpp.o
/home/uday/llvm-project/mlir/lib/Dialect/StandardOps/Ops.cpp: In member function ‘virtual mlir::PatternMatchResult {anonymous}::ViewOpShapeFolder::matchAndRewrite(mlir::ViewOp, mlir::PatternRewriter&) const’:
/home/uday/llvm-project/mlir/lib/Dialect/StandardOps/Ops.cpp:2575:14: warning: variable ‘dynamicOffsetOperandCount’ set but not used [-Wunused-but-set-variable]
 2575 |     unsigned dynamicOffsetOperandCount = 0;
`

Reviewers: rriddle, mehdi_amini, ftynse

Reviewed By: ftynse

Subscribers: jpienaar, burmako, shauheen, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71922