Extract FindAvailablePtrLoadStore out of FindAvailableLoadedValue. NFCI

Summary:
Extract FindAvailablePtrLoadStore out of FindAvailableLoadedValue.
Prepare for upcoming change which will do phi-translation for load on
phi pointer in jump threading SimplifyPartiallyRedundantLoad.

This is in preparation for https://reviews.llvm.org/D30543

Reviewers: efriedma, sanjoy, davide, dberlin

Reviewed By: davide

Subscribers: junbuml, davide, llvm-commits

Differential Revision: https://reviews.llvm.org/D30524

llvm-svn: 298216