[VPlan] Simplify live-ins if they are SCEVConstant.
The legacy cost model in some parts checks if any of the operands are
constants via SCEV. Update VPlan construction to replace live-ins that
are constants via SCEV with such constants. This means VPlans (and
codegen) reflects what we computing the cost of and removes another case
where the legacy and VPlan cost model diverged.
Fixes https://github.com/llvm/llvm-project/issues/105722.
