[LoopVectorize] Use overflow-check analysis to improve tail-folding.

This work follows on from D142109 and addresses a possible regression
when we know the loop iteration counter cannot overflow.

When we know the overflow-check always evaluates to false, it's better to
use the other style of tail folding where it assumes a runtime check was
added, because that avoids having to calculate a modified trip-count.

Reviewed By: paulwalker-arm

Differential Revision: https://reviews.llvm.org/D142894