Handle successor's PHI node correctly when flattening CFG merges two if-regions

Summary:
FlattenCFG merges two 'if' basicblocks by inserting one basicblock
to another basicblock. The inserted basicblock can have a successor
that contains a PHI node whoes incoming basicblock is the inserted
basicblock. Since the existing code does not handle it, it becomes
a badref.

if (cond1)
  statement
if (cond2)
  statement
successor - contains PHI node whose predecessor is cond2

-->
if (cond1 || cond2)
  statement
(BB for cond2 was deleted)
successor - contains PHI node whose predecessor is cond2 --> bad ref!

Author: Jaebaek Seo

Reviewers: asbirlea, kuhar, tstellar, chandlerc, davide, dexonsmith

Reviewed By: kuhar

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68032

llvm-svn: 372989