[Local] Make combineAAMetadata() more principled (#122091) This moves combineAAMetadata() into Local and implements it via a new AAOnly flag, which will intersect only AA metadata and keep other known metadata. The existing KnownIDs list is dropped, because it is redundant with the switch in combineMetadata(), which already drops unknown metadata. I tried a few variants of this, and ultimately went with the AAOnly flag because this way we make an explicit choice for each metadata kind supported by combineMetadata(), and ignoring the flag gives you conservatively correct behavior. I checked that the memcpy tests still pass if we adjust the logic for MD_memprof/MD_callsite to drop the metadata instead of arbitrarily picking one. Fixes https://github.com/llvm/llvm-project/issues/121495.
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