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

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

This needs a rebase. I think this looks basically fine, but I am somewhat concerned about the test coverage. The ExpandMemCmp pass produces IR instructions that are supposed to produce a good lowering on a given target, and what this patch does is convert these to plain IR tests -- that matches our usual testing model, but I also think that it makes it very hard to verify that the produced asm will be good and won't regress based on changes in the backend.

For that reason, I think it would make sense to switch some of the tests (like AArch64/memcmp.extra.ll) to use something like `opt -S -passes=expand-memcmp | llc`. 

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