AMDGPU: Directly implement computeKnownBits for workitem intrinsics

Currently metadata is inserted in a late pass which is lowered
to an AssertZext. The metadata would be more useful if it was
inserted earlier after inlining, but before codegen.

Probably shouldn't change anything now. Just replacing the
late metadata annotation needs more work, since we lose
out on optimizations after these are lowered to CopyFromReg.

Seems to be slightly better than relying on the AssertZext from the
metadata. The test change in cvt_f32_ubyte.ll is a quirk from it using
-start-before=amdgpu-isel instead of running the usual codegen
pipeline.