[LAA] Correctly return a half-open range in expandBounds

[jmolloy]

Summary

This is a latent bug that's been hanging around for a while. For a loop-invariant
pointer, expandBounds would return the range {Ptr, Ptr}, but this was interpreted
as a half-open range, not a closed range. So we ended up planting incorrect
bounds checks. Even worse, they were tautological, so we ended up incorrectly
executing the optimized loop.

[sbaranga]

lib/Analysis/LoopAccessAnalysis.cpp

1944

If we're now always calling expandCodeFor(..), is it still worth checking above if NewPtr is in the loop body?

[jmolloy]

lib/Analysis/LoopAccessAnalysis.cpp

1944

We're only calling expandCodeFor unconditionally on ScPlusOne; I suppose if we're expecting instcombine to clean up from that we should also expect it to clean up expandCodeFor(Sc), but I don't see a need to generate worse code?

[sbaranga]

LGTM

lib/Analysis/LoopAccessAnalysis.cpp

1944

Ok, makes sense.

[jmolloy]

r299526, thanks!