[PowerPC] Remove InstAlias definitions that cause incorrect assembly

In rL283190, I added some InstAlias definitions to generate extended mnemonics
for some uses of the XXPERMDI instruction. However, when the assembler matches
these extended mnemonics, it matches the new instruction in situations where it
should match the old one.
This patch removes these definitions and accomplishes that by defining these
mnemonics with additional instructions that are isCodeGenOnly.

Fixes PR31127.

llvm-svn: 287765