i965: Add support for instruction compaction.

i965: Add support for instruction compaction.

This reduces program size by using some smaller encodings for common bit
patterns in the Gen ISA, with the hope of making programs fit in the
instruction cache better.

v2: Use larger bitshifts for the uncompressed field setups, in line with the
    way it's described in the spec.  Consistently name a brw_compile "p" like
    all other code.  Add a couple more tests.  Consistently call things
    "compacted" not "compressed" (which is a different feature).  Drop the
    explicit check for not compacting SENDs, which is unjustified and already
    implied by our lack of support for immediate values.

Reviewed-by: Paul Berry <stereotype441@gmail.com>