[AArch64] Prepare for changes to STEP_VECTOR.

Rewrite patterns to assume that the operand of STEP_VECTOR is a
constant. The old patterns will stop working when the operand is changed
from a Constant to a TargetConstant. (See D105673.)

Add test coverage for certain patterns that weren't exercised by
existing regression tests.

Differential Revision: https://reviews.llvm.org/D105847