[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.

Author: DamonFool

mlir/lib/Conversion/ComplexToStandard/ComplexToStandard.cpp

@@ -526,7 +526,7 @@ Value evaluatePolynomial(ImplicitLocOpBuilder &b, Value arg,
   auto argType = mlir::cast<FloatType>(arg.getType());
   Value poly =
       b.create<arith::ConstantOp>(b.getFloatAttr(argType, coefficients[0]));
-  for (int i = 1; i < coefficients.size(); ++i) {
+  for (unsigned i = 1; i < coefficients.size(); ++i) {
     poly = b.create<math::FmaOp>(
         poly, arg,
         b.create<arith::ConstantOp>(b.getFloatAttr(argType, coefficients[i])),