[LV] Check VPValue operand instead of Cost::isUniformAfterVec (NFC).

ILV::scalarizeInstruction still uses the original IR operands to check
if an input value is uniform after vectorization.

There is no need to go back to the cost model to figure that out, as the
information is already explicit in the VPlan. Just check directly
whether the VPValue is defined outside the plan or is a uniform
VPReplicateRecipe.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D114253