Put instruction names into an indexed string table on the side, removing a pointer...

Put instruction names into an indexed string table on the side, removing a pointer from MCInstrDesc.

Make them accessible through MCInstrInfo. They are only used for debugging purposes so this doesn't
have an impact on performance. X86MCTargetDesc.o goes from 630K to 461K on x86_64.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@150245 91177308-0d34-0410-b5e6-96231b3b80d8