[PowerPC] Code cleanup. Remove instructions that were withdrawn from Power 9.

The following set of instructions was originally planned to be added for Power 9
and so code was added to support them. However, a decision was made later on to
withdraw support for these instructions in the hardware.
xscmpnedp
xvcmpnesp
xvcmpnedp
This patch removes support for the instructions that were not added.

Differential Revision: https://reviews.llvm.org/D43641

llvm-svn: 325918