[llvm] [MemCpyOpt] support offset slices for performStackMoveOptzn and processMemCpy (PR #176436)

[Jameson Nash via llvm-commits]

vtjnash wrote:

@aeubanks is there any other details you can provide about what was miscompiled about that command? downloading a random sysroot to reproduce feels like a bit large of a step to take. Is that by chance an older c++ stdlib? (the other report of miscompile also only occurred with old stdlibs)

https://github.com/llvm/llvm-project/pull/176436