[PowerPC] Generate inlined quadword lock free atomic operations via AtomicExpand

[PowerPC] Generate inlined quadword lock free atomic operations via AtomicExpand

This patch uses AtomicExpandPass to implement quadword lock free atomic operations. It adopts the method introduced in https://reviews.llvm.org/D47882, which expand atomic operations post RA to avoid spilling that might prevent LL/SC progress.

Reviewed By: jsji

Differential Revision: https://reviews.llvm.org/D103614