[AArch64][SVE] Add patterns for unpredicated load/store to frame-indices.

This patch also fixes up a number of cases in DAGCombine and
SelectionDAGBuilder where the size of a scalable vector is used in a
fixed-width context (thus triggering an assertion failure).

Reviewers: efriedma, c-rhodes, rovka, cameron.mcinally

Reviewed By: efriedma

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71215