[LoopFlatten] Fix IV increment use count

The add from the IV in the inner loop was always checking for 2 uses,
the phi and the compare. The compare could be based on the phi though,
leaving one valid use of the compare. In the testcase we could be left
with the phi and a lcssa phi as the two users, invalidly allowing
flattening where we shouldn't.

Fixes 58441

Differential Revision: https://reviews.llvm.org/D138404