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

If all the operands to a phi node are getelementptr, 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 new getelementptr
should be the merged debug locations of the phi node arguments.

Patch 4 of 8 for D26256.  Folding of a getelementptr operation.

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

llvm-svn: 289684