[CostModel] get type-based cost for vector intrinsics directly

This is NFC-intended (the reductions and other intrinsics have
tests that should provide coverage) - trying to untangle the
mess that has formed here.

We were stripping the arguments off of the given
"IntrinsicCostAttributes" and then semi-recursively
calling back into getIntrinsicInstrCost() only to
then call getTypeBasedIntrinsicInstrCost(), so make
that call directly instead.