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.
71f7b972c3