[GlobalISel][AArch64] Legalize/select G_(S/Z/ANY)_EXT for v8s8s

[GlobalISel][AArch64] Legalize/select G_(S/Z/ANY)_EXT for v8s8s

This adds legalization for G_SEXT, G_ZEXT, and G_ANYEXT for v8s8s.

We were falling back on G_ZEXT in arm64-vabs.ll before, preventing us from
selecting the @llvm.aarch64.neon.sabd.v8i8 intrinsic.

This adds legalizer support for those 3, which gives us selection via the
importer. Update the relevant tests (legalize-ext.mir, select-int-ext.mir) and
add a GISel line to arm64-vabs.ll.

Differential Revision: https://reviews.llvm.org/D60881

llvm-svn: 358715