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

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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@299526 91177308-0d34-0410-b5e6-96231b3b80d8