[InstCombine] Perform memset -> load forwarding

InstCombine does some basic store to load forwarding. One case it
currently misses is the case where the store is actually a memset.
This patch adds support for this case. This is a minimal
implementation that only handles a load at the memset base address,
without an offset.

GVN is already capable of performing this optimization. Having it
in InstCombine can help with phase ordering issues, similar to the
existing store to load forwarding.

Differential Revision: https://reviews.llvm.org/D137323