[MemCpyOpt] Optimize MemoryDef insertion

When converting a store into a memset, we currently insert the new
MemoryDef after the store MemoryDef, which requires all uses to be
renamed to the new def using a whole block scan. Instead, we can
insert the new MemoryDef before the store and not rename uses,
because we know that the location is immediately overwritten, so
all uses should still refer to the old MemoryDef. Those uses will
get renamed when the old MemoryDef is actually dropped, which is
efficient.

I expect something similar can be done for some of the other MSSA
updates in MemCpyOpt. This is an alternative to D107513, at least
for this particular case.

Differential Revision: https://reviews.llvm.org/D107702