[clang] [llvm] [InstCombine] remove dead loads, such as memcpy from undef (PR #143958)

[Jameson Nash via llvm-commits]

================

----------------
vtjnash wrote:

I've tried to adjust most of those to avoid alloca, though it is triggering some sort of bug with `simplify_before_foldAndOfICmps` which I'll have to investigate later

https://github.com/llvm/llvm-project/pull/143958