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