[IndVarSimplify][Dwarf] When widening the IV increment, correctly set the debug loc.

When indvars widened an induction variable, the debug location for the loop
increment computation was incorrectly set equal to the debug loc of the loop
latch terminator.

This patch fixes the issue by propagating the correct location from the
original loop increment instruction to the new widened increment.

Differential Revision: https://reviews.llvm.org/D25872

llvm-svn: 285083