i965: Accurately bail on SIMD16 compiles.

Ideally, we'd like to never even attempt the SIMD16 compile if we could
know ahead of time that it won't succeed---it's purely a waste of time.
This is especially important for state-based recompiles, which happen at
draw time.

The fragment shader compiler has a number of checks like:

   if (dispatch_width == 16)
      fail("...some reason...");

This patch introduces a new no16() function which replaces the above
pattern.  In the SIMD8 compile, it sets a "SIMD16 will never work" flag.
Then, brw_wm_fs_emit can check that flag, skip the SIMD16 compile, and
issue a helpful performance warning if INTEL_DEBUG=perf is set.  (In
SIMD16 mode, no16() calls fail(), for safety's sake.)

The great part is that this is not a heuristic---if the flag is set, we
know with 100% certainty that the SIMD16 compile would fail.  (It might
fail anyway if we run out of registers, but it's always worth trying.)

v2: Fix missing va_end in early-return case (caught by Ilia Mirkin).

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Chris Forbes <[email protected]> [v1]
Reviewed-by: Ian Romanick <[email protected]> [v1]
Reviewed-by: Eric Anholt <[email protected]>

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