[LoopIdiom] Preserve alias information for memset_pattern

TBAA/NoAlias/AliasScope and other information is currently preserved
when upgrading to a memcpy/memset. However, this is missing when upgrading to
the macOS memset_pattern function. This adds the same alias information preservation
to memset_pattern

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D152934