[llvm] [MemCpyOpt] Continue merging memset with unrelated clobber (PR #89550)

[via llvm-commits]

XChy wrote:

> Compile-time impact: https://llvm-compile-time-tracker.com/compare.php?from=e32c4dfefcd1d54eb8f353f6fa08ef6f06d0fcc4&to=d63f94bf92b103c0de6d8c1e9cc00ed5c5a58b06&stat=instructions:u

Stage2 compile-time impact looks reasonable to me.

https://github.com/llvm/llvm-project/pull/89550