[PowerPC] Fix memory corruption in AsmParser

As pointed out by Evgeniy Stepanov, assigning a std::string temporary
to a StringRef is not a good idea.  Rework MatchRegisterName to avoid
using the .lower routine.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181192 91177308-0d34-0410-b5e6-96231b3b80d8
Free the exception object. Should fix the vg bots.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181195 91177308-0d34-0410-b5e6-96231b3b80d8