[llvm] Move ExpandMemCmp and MergeIcmp to the middle end (PR #77370)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

I don't really know what the issue was, exactly.  The review doesn't say, and I don't have any further information.  We may want to disable memcmp expansion when msan is enabled, to allow the specialized diagnostics to trigger.

I am a little concerned about the semantics of uninitialized memory... currently, we assume that memcmp stops comparing after the first non-equal byte.  And the current expansion doesn't try to freeze the loaded values.  (See LibCallSimplifier::optimizeStrCmp .)

https://github.com/llvm/llvm-project/pull/77370