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

[Gabriel Baraldi via llvm-commits]

gbaraldi wrote:

> I'm surprised by the volume of test changes

I just copied/ported all the tests we had that tested memcmp expansion, which apparently was quite a bit. Though I guess there’s now quite a bit of repetition.

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