[InstCombine] Folding of a compare with RHS const should merge debug locations

If all the operands to a phi node are compares that have a RHS constant,
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 op
should be the merged debug locations of the phi node arguments.

Patch 8 of 8 for D26256.  Folding of a compare that has a RHS constant.

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

llvm-svn: 289704