[AArch64][GlobalISel] Select trn1 and trn2

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.

Differential Revision: https://reviews.llvm.org/D81182

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