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

If all the operands to a phi node are a binop with 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 7 of 8 for D26256.  Folding of a binop with RHS constant.

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

llvm-svn: 289699