Port Swift's merge function pass to llvm: merging functions that differ in constants (#68235)

See RFC for details:
https://discourse.llvm.org/t/rfc-for-moving-swift-s-merge-function-pass-to-llvm/73778



We will need to refactor extension to FunctionComparator/FunctionHash to
StructuralHash. This patch adds a new pass which is ported from Swift,
and will need to discuss on how to migrate Swift’s pass over after we
land this in llvm.

Create this PR to get some early review on the patch.

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Co-authored-by: Manman Ren <mren@meta.com>