[SVE] Lower fixed-length floating point loads and stores to integer variants.

[SVE] Lower fixed-length floating point loads and stores to integer variants.

There's no advatange to emitting floating point scalable accesses,
whereas by lowering them to integer variants we can benefit from
several combines that seek to replace explicit extends/truncates
with extending/truncating accesses.

Differential Revision: https://reviews.llvm.org/D132393