[NFCI][IndVars] rewriteLoopExitValues(): nowadays SCEV should not change `GEP` base pointer

Currently/previously, while SCEV guaranteed that it produces the same value,
the way it was produced may be illegal IR, so we have an ugly check that
the replacement is valid.

But now that the SCEV strictness wrt the pointer/integer types has been improved,
i believe this invariant is already upheld by the SCEV itself, natively.

I think we should add an assertion, wait for a week, and then, if all is good,
rip out all this checking.
Or we could just do the latter directly i guess.

This reverts commit rL127839.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D108043