[LoopFlatten] Fix crash if the inner loop trip count comes from a sext instruction.

If we look through a truncate in matchLinearIVUser, it's possible
we find a sext/zext instruction that didn't come from widening.
This will fail the MatchedItCount->getType() == InnerInductionPHI->getType()
assertion.

Fix this by checking that we did not look through a truncate already.

Reviewed By: SjoerdMeijer

Differential Revision: https://reviews.llvm.org/D127149