[llvm] [RegAlloc] Remove default restriction on non-trivial rematerialization (PR #159211)

[Luke Lau via llvm-commits]

lukel97 wrote:

This should be ready for review now, I've rebased and rerun the results on rva23u64 -O3 and arm64-apple-darwin -O3 and there was virtually no change to the previous results in number of registers spilled/reloaded. I've also updated the PR description to clarify that we actually previously had non-trivial remat, and to mention the other work that went into untangling the API.

https://github.com/llvm/llvm-project/pull/159211