[SVE][CodeGen] Legalisation of EXTRACT_VECTOR_ELT for scalable vectors

This patch changes SplitVecOp_EXTRACT_VECTOR_ELT to work correctly
for scalable vectors and also fixes an a bug in DAGCombiner where
the scalable property is dropped in visitTRUNCATE when attempting
to fold an extract + a truncate.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D85754