[VPlan] Use isUniformAfterVec in VPReplicateRecipe::execute.

I was unable to find a case where this actually changes generated code,
but it enables the bug fix in D144434. It also brings codegen in line
with the handling of stores to uniform addresses in the cost model
(D134460).

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D144491