[MemorySSA] Consider callsite args for hashing and equality.

We use a `DenseMap<MemoryLocOrCall, MemlocStackInfo>` to keep track of
prior work when optimizing uses in MemorySSA. Because we weren't
accounting for callsite arguments in either the hash code or equality
tests for `MemoryLocOrCall`s, we optimized uses too aggressively in
some rare cases.

Fix by Daniel Berlin.

Should fix PR36883.

llvm-svn: 328748