[LCSSA] Post-process PHI-nodes created by SSAUpdate when constructing LCSSA form.

Summary:
SSAUpdate might insert PHI-nodes inside loops, which can break LCSSA
form unless we fix it up.

This fixes PR28424.

Reviewers: sanjoy, chandlerc, hfinkel

Subscribers: uabelho, llvm-commits

Differential Revision: http://reviews.llvm.org/D21997

llvm-svn: 275883