[SLPVectorizer][SVE] Skip scalable-vector instructions before vectorizeSimpleInstructions.

For scalable type, the aggregated size is unknown at compile-time.
Skip instructions with scalable type to ensure the list of instructions
for vectorizeSimpleInstructions does not contains any scalable-vector instructions.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D87550