gallivm: handle SAMPLE opcode in aos sampling

gallivm: handle SAMPLE opcode in aos sampling

This is just a very limited version, in particular sampler and sampler view
index must be the same. It cannot handle any modifiers neither.
Works much the same as soa version otherwise, to figure out the target we
need to store the sampler view dcls.
While here, also handle (no-op) RET and get rid of a couple bogus deprecated
comments.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>