[SVE] Bail from VectorUtils heuristics for scalable vectors

Bail from maskIsAllZeroOrUndef and maskIsAllOneOrUndef prior to iterating over the number of
elements for scalable vectors.

Assert that the mask type is not scalable in possiblyDemandedEltsInMask .

Assert that the types are correct in all three functions.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D87424