[AArch64] Move BSL generation to lowering. (#151855)

It is generally better to allow the target independent combines before
creating AArch64 specific nodes (providing they don't mess it up). This
moves the generation of BSL nodes to lowering, not a combine, so that
intermediate nodes are more likely to be optimized. There is a small
change in the constant handling to detect legalized buildvector
arguments correctly.

Fixes #149380 but not directly. #151856 contained a direct fix for
expanding the pseudos.

0 files changed, 0 insertions, 0 deletions