[llvm] [clang-tools-extra] [clang] Add out-of-line-atomics support to GlobalISel (PR #74588)

[Matt Arsenault via cfe-commits]

arsenm wrote:

>  // At -O0, fast-regalloc cannot cope with the live vregs necessary to

This sounds extremely unsound 

https://github.com/llvm/llvm-project/pull/74588