[PowerPC] Fix up memory ordering after combining BV to a load

The combiner for BUILD_VECTOR that merges consecutive
loads into a wide load had two issues:

- It didn't check that the input loads all have the
  same input chain
- It didn't update nodes that are chained to the original
  loads to be chained to the new load

This caused issues with bootstrap when
3c4d2a03 was committed.
This patch fixes the issue so it can unblock this commit.

Differential revision: https://reviews.llvm.org/D140046