nir: Add texture opcodes and source types for multisample compression

nir: Add texture opcodes and source types for multisample compression

Intel hardware does a form of multisample compression that involves an
auxilary surface called the MCS.  When an MCS is in use, you have to first
sample from the MCS with a special opcode and then pass the result of that
operation into the next sample instrucion.  Normally, we just do this
ourselves in the back-end, but we want to expose that functionality to NIR
so that we can use MCS values directly in NIR-based blorp.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>