[GlobalISel][AArch64] Add generic legalization rule for extends

This adds a legalization rule for G_ZEXT, G_ANYEXT, and G_SEXT which allows
extends whenever the types will fit in registers (or the source is an s1).

Update tests. Add GISel checks throughout all of arm64-vabs.ll,
where we now select a good portion of the code. Add GISel checks to
arm64-subvector-extend.ll, which has a good number of vector extends in it.

Differential Revision: https://reviews.llvm.org/D60889

llvm-svn: 359222