[AIX] Fixed "comparison of unsigned expression >= 0 is always true" gcc warnings.

Summary:

fixed a  Fixed "comparison of unsigned expression >= 0 is always true" gcc warnings.
http://lab.llvm.org:8011/#/builders/5/builds/2407/steps/2/logs/stdio

the error caused by patch https://reviews.llvm.org/D92398

Author: diggerlin

llvm/lib/Target/PowerPC/PPCMachineFunctionInfo.cpp

@@ -67,7 +67,7 @@ bool PPCFunctionInfo::isLiveInZExt(Register VReg) const {
 
 void PPCFunctionInfo::appendParameterType(ParamType Type) {
   uint32_t CopyParamType = ParameterType;
-  unsigned Bits = 0;
+  int Bits = 0;
 
   // If it is fixed type, we only need to increase the FixedParamNum, for
   // the bit encode of fixed type is bit of zero, we do not need to change the