[SCEV] Turn check in createSimpleAffineAddRec to assertion. (NFC)

Accum is guaranteed to be defined outside L (via Loop::isLoopInvariant
checks above). I think that should guarantee that the more powerful
ScalarEvolution::isLoopInvariant also determines that the value is loop
invariant.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D114634