Same idea as for zip, uzp, etc. Teach the post-legalizer combiner to recognize G_SHUFFLE_VECTORs that are trn1/trn2 instructions.
- Add G_TRN1 and G_TRN2
- Port mask matching code from AArch64ISelLowering
- Produce G_TRN1 and G_TRN2 in the post-legalizer combiner
- Select via importer
Add select-trn.mir to test selection.
Add postlegalizer-combiner-trn.mir to test the combine. This is similar to the existing arm64-trn test.
Note that both of these tests contain things we currently don't legalize.
I figured it would be easier to test these now rather than later, since once we legalize the G_SHUFFLE_VECTORs, it's not guaranteed that someone will update the tests.