[ISEL] Canonicalize STEP_VECTOR to LHS if RHS is a splat.

This helps recognise patterns where we're trying to match STEP_VECTOR
patterns to INDEX instructions that take a GPR for the Start/Step.

The reason for canonicalising this operation to the LHS is
because it will already be canonicalised to the LHS if the RHS
is a constant splat vector.

Reviewed By: paulwalker-arm

Differential Revision: https://reviews.llvm.org/D118459