[InstCombine] When folding casts through a phi node merge the debug locations

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

Patch 6 of 8 for D26256.  Folding of a cast operation.

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

llvm-svn: 289693