[InstCombine] Folding loads through a phi node should merge the debug locations

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

Patch 5 of 8 for D26256.  Folding of a load operation.

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

llvm-svn: 289688