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

[Gabriel Baraldi via llvm-commits]

gbaraldi wrote:

So what confused me about that concern was that we don't care about this for memcpy for example, where if instcombine decides to do the inline expansion, it just happens, asan/msan notwidthstanding.

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