[llvm] [LowerMemIntrinsics] Use i8 GEPs in memcpy/memmove lowering (PR #112707)

[Fabian Ritter via llvm-commits]

ritter-x2a wrote:

The failure on `clang-hip-vega20` looks very much like [these intermittent failures again](https://github.com/llvm/llvm-project/issues/98491). I'll put debugging those higher on my todo list...
A couple of other recent builds with a similar error:
- https://lab.llvm.org/buildbot/#/builders/123/builds/7933
- https://lab.llvm.org/buildbot/#/builders/123/builds/7865

https://github.com/llvm/llvm-project/pull/112707