[LSR] Use evaluateAtIteration in lsr-term-fold

[LSR] Use evaluateAtIteration in lsr-term-fold

This is a follow up to one of the side discussions on D146429.  There are two semantic changes contained here.

The motivation for the change to the legality condition introduced in D146429 comes from the fact that we only check the post-inc form. As such, as long as the values of the post-inc variable don't self wrap, it's actually okay if we wrap past the starting value of the pre-inc IV.

Second, Nikic noticed during review that the test changes changed behavior for TC=0 (i.e. N=0 in the tests).  On more careful inspection, it became apparent that the previous manual expansion code was incorrect in the case where the primary IV could wrap without poison, and started with the limit value (i.e. i8 post-inc starts at 255 for 0 exit test, implying pre-inc starts with 0).  See @wrap_around test for an example of the (previous) miscompile.

Differential Revision: https://reviews.llvm.org/D146457