[SROA] Fold a PHI node if all its incoming values are the same

Summary:
Fixes PR20425.

During slice building, if all of the incoming values of a PHI node are the same, replace the PHI node with the common value. This simplification makes alloca's used by PHI nodes easier to promote.

Test Plan: Added three more tests in phi-and-select.ll

Reviewers: nlewycky, eliben, meheff, chandlerc

Reviewed By: chandlerc

Subscribers: zinovy.nis, hfinkel, baldrick, llvm-commits

Differential Revision: http://reviews.llvm.org/D4659

llvm-svn: 216299