[Clang][PowerPC] Support constraint 'wi' in asm

  From the gcc manual, we can see that the specific limit of wi inline asm is “FP or VSX register to hold 64-bit integers for VSX insns or NO_REGS”. The link is https://gcc.gnu.org/onlinedocs/gcc-8.2.0/gcc/Machine-Constraints.html#Machine-Constraints. We should accept this constraint.

Reviewed By: jsji

Differential Revision: https://reviews.llvm.org/D53265

llvm-svn: 345809