This patch just adds more tests for cases where we extract elements
from a scalable vector and re-insert them into a new fixed-width vector:
CodeGen/AArch64/sve-fixed-length-reshuffle.ll
Some of these tests exercise the AArch64TargetLowering::ReconstructShuffle
code path.
Do we also want a test for extract elements 0 and 3, and inserting that into elements 0 and 1?