[AArch64][GlobalISel] Mark some vector G_ABS cases as legal
Each of the cases marked as legal here have an imported pattern in
AArch64GenGlobalISel.inc. So, if we mark them as legal, we get selection for
Technically this is only supposed to happen if we have NEON support. But, we
fall back if we don't have that in the legalizer right now. I suppose it'd be
better to have a FIXME so we can write the testcase when the time comes.
(Plus, it'd just fall back in selection if NEON isn't available, so it's not
*wrong*, I guess?)
This fixes some fallbacks in the test suite.
(Also use isScalar from LegalityPredicates.cpp while we're here just to tidy
things a little bit.)
Differential Revision: https://reviews.llvm.org/D100916