AMDGPU: Cleanup SMRD buffer selection

The usage of the Imm out argument from SelectSMRDOffset is pretty
confusing. Stop trying to reject CI immediates in the case where the
offset field can be used. It's not an illegal way to encode the
immediate, so just prefer the better encoding pattern with
AddedComplexity.

We probably don't even really need the different opcodes for the
different offset types anymore, but that will be more work to cleanup.

The SMRD non-buffer load patterns could also use a cleanup to be done
separately.