[ISEL] Move trivial step_vector folds to FoldConstantArithmetic.

Given that step_vector is practically a constant, doing this early
helps with DAGCombine folds that happen before type legalization.

There is currently no way to test this happens earlier, although existing
tests for step_vector folds continue protect the folds happening at all.

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D117863