[MemorySSA] Remove PerformedPhiTranslation flag

I believe this is no longer necessary, as the underlying problem
has been fixed in a different way: Nowadays, we will adjust the
location size to beforeOrAfterPointer() if the pointer is not loop
invariant. This makes merging results translated across loop
backedges safe.

The two tests in phi-translation.ll show an improvement while still
being correct: The loads in the loop no longer alias with noalias
pointers, but still alias with the store in the entry block (which
they originally did not -- this is the bug that
PerformedPhiTranslation originally fixed).

Differential Revision: https://reviews.llvm.org/D133404