Recommit "[mips] Fix atomic compare and swap at O0."

This time with the missing files.

Similar to PR/25526, fast-regalloc introduces spills at the end of basic
blocks. When this occurs in between an ll and sc, the store can cause the
atomic sequence to fail.

This patch fixes the issue by introducing more pseudos to represent atomic
operations and moving their lowering to after the expansion of postRA
pseudos.

This resolves PR/32020.

Thanks to James Cowgill for reporting the issue!

Reviewers: slthakur

Differential Revision: https://reviews.llvm.org/D30257

llvm-svn: 296134