[MemorySSAUpdater] Update Phi operands after trivial Phi elimination

Bug fix for PR37445. The underlying problem and its fix are similar to PR37808.
The bug lies in MemorySSAUpdater::getPreviousDefRecursive(), where PhiOps is
computed before the call to tryRemoveTrivialPhi() and it ends up being out of
date, pointing to stale data. We have now turned each of the PhiOps into a
TrackingVH<MemoryAccess>.

Differential Revision: https://reviews.llvm.org/D49425

llvm-svn: 337680