Fix out-of-order stepping behavior in programs with hoisted constants.

When the Constant Hoisting pass moves expensive constants into a
common block, it would assign a debug location equal to the last use
of that constant. While this is certainly intuitive, it places the
constant in an out-of-order location, according to the debug location
information. This produces out-of-order stepping when debugging
programs affected by this pass.

This patch creates in-order stepping behavior by merging the debug
locations for hoisted constants, and the new insertion point.

Patch by Matthew Voss!

Differential Revision: https://reviews.llvm.org/D38088

llvm-svn: 317827