[AArch64][GlobalISel] Select zip1 and zip2

Port the code to recognize a zip1/zip2 shuffle mask from AArch64ISelLowering
and put it into the post-legalizer combiner.

Add G_ZIP1 and G_ZIP2 to AArch64InstrGISel.td and hook them up as equivalent
nodes to AArch64zip1 and AArch64zip2. This allows us to select them.

Minor code size improvements for SPECINT2000 at -O3 on 197.parser, 252.eon, and
186.crafty.

Differential Revision: https://reviews.llvm.org/D80969