LiveDebugValues gives variable locations to blocks, but it should also take away. There are various circumstances where a variable location is known until a loop backedge with a different location is detected. In those circumstances, where there's no agreement on the variable location, it should be undef / removed, otherwise we end up picking a location that's valid on some loop iterations but not others.
However, LiveDebugValues doesn't currently do this, see the new testcase attached. Without this patch, the location of !3 is assumed to be %bar through the loop. Once it's added to the In-Locations list, it's never removed, even though the later dbg.value(0... of !3 makes the location un-knowable.
This patch checks during block-location-joining to see whether any previously-present locations have been removed in a predecessor. If they have, the live-ins have changed, and the block needs reprocessing. Similarly, in transferTerminator, assign rather than |= the Out-Locations after processing a block, as we may have deleted some previously valid locations. This will mean that LiveDebugValues performs more propagation -- but that's necessary for it being correct.
In the two tests that change, in pieces.ll some spurious variable locations are now no longer propagated. In fission-ranges.ll, we now notice that the "a" / !14 variable is not valid for most of the function, instead a backedge with a mismatching location invalidates its location early. The control flow in the newly added test is slightly more complex than it needs to be (loop2 isn't necessary) due to other bugs.
On a clang-3.4 build a tiny number of variable locations get dropped, however the percentage of scope-bytes-covered falls from 46.5% to 45.2%. IMO this is good: those are invalid locations.