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 no longer is consistent with the
original source code. In order to ensure correct stepping in the
debugger, the debug location for sunk instructions is either merged
with the insertion point or erased if the target successor block is
empty.

Originally submitted as r318679, revised to fix sanitizer failure and
improve testing.

Patch by Matthew Voss!

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

llvm-svn: 320216