[mlir] Fix -Wsign-compare in ComplexToStandard.cpp (NFC)
/llvm-project/mlir/lib/Conversion/ComplexToStandard/ComplexToStandard.cpp:529:21:
error: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Werror,-Wsign-compare]
529 | for (int i = 1; i < coefficients.size(); ++i) {
| ~ ^ ~~~~~~~~~~~~~~~~~~~
1 error generated.
