[AMDGPU] Implement ISel for V_SAT_PK_U8_I16

[Sisyph]

It appears we have no way to select into this instruction.
It likely could be selected from llvm.sadd.sat

See https://reviews.llvm.org/D135478

[llvmbot]

@llvm/issue-subscribers-backend-amdgpu

[jayfoad]

It likely could be selected from llvm.sadd.sat

I don't see how. This instruction does not do an add. It is more like a saturating truncate from 16-bit signed to 8-bit unsigned.

[Pierre-vh]

I'm looking at this and will submit a patch if I finish it - assigning to myself in the meantime