[llvm] [MemCpyOpt] Drop dead `memmove` calls on `memset`'d source data (PR #101930)

[Antonio Frighetto via llvm-commits]

antoniofrighetto wrote:

> precomitting the test would be nice so we can see the effect of this change

Tests are already in a separate commit (https://github.com/llvm/llvm-project/pull/101930/commits/2573b26d9dc2dcb53fe0492eccecd2f6f8486295).

> also could you run this through https://llvm-compile-time-tracker.com/?

Compile-time looks OK: https://llvm-compile-time-tracker.com/compare.php?from=2573b26d9dc2dcb53fe0492eccecd2f6f8486295&to=fd47d7f80291f5384c2ace760463cd39791f81ff&stat=instructions:u.

https://github.com/llvm/llvm-project/pull/101930