[PowerPC] Improve kill flag computation and add verification after MI peephole

The MI Peephole pass has grown to include a large number of transformations over the years. Many of the transformations require re-computation of kill flags but don't do a good job of re-computing them. This causes us to have very common failures when the compiler is built with expensive checks. Over time, we added and augmented a function that is supposed to go and fix up kill flags after each transformation but we keep missing cases.
This patch does the following:
- Removes the function to re-compute kill flags
- Adds LiveVariables to compute and maintain kill flags while transforming code
- Adds re-computation of kill flags for the post-RA peepholes for each block that contains a transformed instruction

Reviewed By: stefanp

Differential Revision: https://reviews.llvm.org/D133103