[EarlyCSE] Don't eagerly optimize MemoryUses

EarlyCSE currently optimizes all MemoryUses upfront. However,
EarlyCSE only actually queries the clobbering memory access for
a subset of uses, namely those where a CSE candidate has already
been identified. Delaying use optimization to the clobber query
improves compile-time in practice.

This change is not NFC because EarlyCSE has a limit on the number
of clobber queries (EarlyCSEMssaOptCap), in which case it falls
back to the defining access. The defining access for uses will now
no longer coincide with the optimized access.

If there are performance regressions from this change, we should
be able to address them by raising this limit.

Differential Revision: https://reviews.llvm.org/D121987