[EarlyCSE] Make MemorySSA memory dependency check more aggressive.

Now that MemorySSA keeps track of whether MemoryUses are optimized, use
getClobberingMemoryAccess() to check MemoryUse memory dependencies since
it should no longer be so expensive.

This is a follow-up change to https://reviews.llvm.org/D25881

llvm-svn: 285080