Fix out-of-order stepping behavior in programs with sunk instructions.

MachineSink attempts to place instructions near the basic blocks where
they are needed.  Once an instruction has been sunk, its location
relative to other instructions is no longer consistent with the
original source code. In order to ensure correct single-stepping and
profiling, the debug location for sunk instructions is either merged
with the insertion point or erased if the target successor block is
empty.

Patch by Matthew Voss!

Differential Revision: https://reviews.llvm.org/D39933

llvm-svn: 318679