[NFC][MLIR] Fix crash in ConvertComplexPow.

Author: TIFitis

flang/lib/Optimizer/Transforms/ConvertComplexPow.cpp

@@ -83,9 +83,7 @@ void ConvertComplexPowPass::runOnOperation() {
         call.setFastmathAttr(fmf);
       powIop.replaceAllUsesWith(call.getResult(0));
       powIop.erase();
-    }
-
-    if (auto powOp = dyn_cast<complex::PowOp>(op)) {
+    } else if (auto powOp = dyn_cast<complex::PowOp>(op)) {
       builder.setInsertionPoint(powOp);
       Location loc = powOp.getLoc();
       auto complexTy = cast<ComplexType>(powOp.getType());