i965/fs: Add stub lowering pass for logical send-message opcodes.

i965/fs: Add stub lowering pass for logical send-message opcodes.

This pass will house ad-hoc lowering code for several send
message-like virtual opcodes that will represent their logically
independent arguments as separate instruction sources rather than as a
single payload blob.  This pass will basically just take the separate
arguments that are supposed to be part of the payload and concatenate
them to construct a message in the form required by the hardware.
Virtual instructions in separate-source form will eventually allow
some simplification of the visitor code and make several
transformations easier like lowering SIMD16 instructions to SIMD8
algorithmically in cases where the hardware doesn't support the former
natively.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>