[AArch64][GlobalSel] Add isel support for a couple vector exts/truncs

[paquette]

Summary

Add support for

• v4s16 <-> v4s32
• v2s64 <-> v2s32

And update tests that use them to show that we generate the correct instructions.

[aemerson]

I think if you're going to have a new separate test, you might as well make it a MIR test for the selector (just use update_mir_test_checks.py) instead of an IR test which already exists in arm64-vcvt_f.ll.

[paquette]

Updated existing .mir test instead of adding a .ll test

[arsenm]

test/CodeGen/AArch64/GlobalISel/select-fp-casts.mir

5–7 ↗

(On Diff #186288)

You shouldn't need the IR section at all in this test. Maybe a pre-commit to remove this?

[paquette]

test/CodeGen/AArch64/GlobalISel/select-fp-casts.mir

5–7 ↗

(On Diff #186288)

Good idea. r353731.

[paquette]

Rebased after removing redundant IR from existing .mir test

[aemerson]

LGTM.