[llvm] Reland: Port Swift's merge function pass to llvm: merging functions that differ in constants (PR #71584)

[Manman Ren via llvm-commits]

manman-ren wrote:

@nikic Hopefully this updated patch is in a reviewable state. As mentioned in the description, this pass was ported over from Swift repo (@eeckstein is the original implementor of this pass), when global merging is enabled, it can provide up to 3.5% code size reduction on top of global machine outliner.

CC @fhahn to make sure the earlier comments are addressed.
Feel free to add other reviewers in order to make sure this PR is adequately reviewed.

https://github.com/llvm/llvm-project/pull/71584