[VPlan] Avoid collecting scalars for SVE

This patch ensures scalars (except for uniforms) are no
longer collected (prior to LVP planning phase) for
scalable vectorization.

This is to avoid the chances of generating scalarized
instructions later (during LVP execute phase) as they
are not supported for scalable vectorization.

Relevant test has also been added.

Differential Revision: https://reviews.llvm.org/D121452