[PowerPC] VSX register support for inline assembly

Summary:
The patch is to add the VSX register support for inline assembly. After this 
patch, we can use VSX register in inline assembly clobber list without error.

Reviewed By: jsji,  nemanjai

Differential Revision: https://reviews.llvm.org/D55192


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@348572 91177308-0d34-0410-b5e6-96231b3b80d8