[LoopPeel] Turn incorrect assert into a check

Summary:
This patch replaces incorrectt assert with a check. Previously it asserts that
if SCEV cannot prove `isKnownPredicate(A != B)`, then it should be able to prove
`isKnownPredicate(A == B)`.

Both these fact may be not provable. It is shown in the provided test:

Could not prove: `{-294,+,-2}<%bb1> !=  0`
Asserting: `{-294,+,-2}<%bb1> == 0`

Obviously, this SCEV is not equal to zero, but 0 is in its range so we cannot
also prove that it is not zero.

Instead of assert, we should be checking the required conditions explicitly.

Reviewers: lebedev.ri, fhahn, sanjoy, fedor.sergeev
Reviewed By: lebedev.ri
Subscribers: hiraditya, zzheng, javed.absar, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76050