[InstCombine] Merge debug locations when folding through a phi node

If all the operands to a phi node are of the same operation, instcombine
will try to pull them through the phi node, combining them into a single
operation.  When it does this, the debug location of the operation should
be the merged debug locations of the phi node arguments.

Patch 2 of 8 for D26256.  Folding of a binary operation.

Differential Revision: https://reviews.llvm.org/D26256

llvm-svn: 289679