[LV] Check if value was already not uniform for previous VF.

If the value was already known to not be uniform for the previous
(smaller VF), it cannot be uniform for the larger VF.

This slightly reduces compile-time, once uniformity checks are becoming
a bit more expensive due to using SCEV rewriting (D148841).

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D151658