[LV] Use VPReplicateRecipe::isUniform instead isUniformAfterVec (NFCI).

In scalarizeInstruction(), isUniformAfterVectorization is used to detect
cases where it is sufficient to always access the first lane. This
should map directly checking whether the operand is a uniform replicate
recipe.

Differential Revision: https://reviews.llvm.org/D116654