[LV] Split off invariance check from isUniform (NFCI).

After 572cfa3f, isUniform now checks VF based uniformity instead of
just invariance as before.

As follow-up cleanup suggested in D148841, separate the invariance check
out and update callers that currently check only for invariance.

This also moves the implementation of isUniform from LoopAccessAnalysis
to LoopVectorizationLegality, as LoopAccesAnalysis doesn't use the more
general isUniform.