[PowerPC] Add some InstAlias for mtspr/mfspr instructions

Summary:

We have defined MTSPR/MFSPR and MTSPR8/MFSPR8, but we only defined
mtspr/mfspr InstAlias for some MTSPR/MFSPR.
This patch is to add the InstAlias definitions for MTSPR8/MFSPR8,
and add the some new mtspr/mfspr InstAlias we may use.

Reviewed By: steven.zhang

Differential Revision: https://reviews.llvm.org/D77531