[InstCombine] Remove memset of undef value

This removes memset with undef char. We already do this for stores
of undef value.

This comes with the caveat that this optimization is not, strictly
speaking, legal for undef values, because we might be overwriting
a poison value. However, our entire load/store model currently still
operates on undef values, so we need to support undef here as well
for internal consistency.

Once https://github.com/llvm/llvm-project/issues/52930 is resolved,
these and related folds can be limited to poison -- I've added
FIXMEs to that effect.

Differential Revision: https://reviews.llvm.org/D124173