[SVE] Don't consider scalable vector types in SLPVectorizerPass::vectorizeChainsInBlock

In vectorizeChainsInBlock we try to collect chains of PHI nodes
that have the same element type, but the code is relying upon
the implicit conversion from TypeSize -> uint64_t. For now, I have
modified the code to ignore PHI nodes with scalable types.

Differential Revision: https://reviews.llvm.org/D83542