[JumpThreading] Perform phi-translation in SimplifyPartiallyRedundantLoad.

Summary:
In case we are loading on a phi-load in SimplifyPartiallyRedundantLoad.
Try to phi translate it into incoming values in the predecessors before
we search for available loads.

This needs https://reviews.llvm.org/D30524

Reviewers: davide, sanjoy, efriedma, dberlin, rengolin

Reviewed By: dberlin

Subscribers: junbuml, llvm-commits

Differential Revision: https://reviews.llvm.org/D30543

llvm-svn: 298217