[PowerPC] Support operand modifier 'x' in inline asm

gcc uses operand modifier 'x' in inline asm for VSX registers.
Without this modifier, instructions which use VSX numbering for their
operands are printed as VMX registers. This patch adds support for the
operand modifier 'x'.

Differential Revision: https://reviews.llvm.org/D52244

llvm-svn: 342882