intel/fs: Implement performance analysis-based SIMD32 heuristic for fragment shaders.

intel/fs: Implement performance analysis-based SIMD32 heuristic for fragment shaders.

The heuristic enables the SIMD32 fragment shader based on whether the
IR performance modeling pass predicts it to have greater throughput
than the SIMD16 and SIMD8 variants of the same shader.  It would be
straightforward to do the same thing in order to control whether
SIMD16 dispatch is enabled, but it's pending additional performance
evaluation.

The INTEL_DEBUG=do32 option is left around in order to force the
SIMD32 shader to be used regardless of the result of the heuristic,
since it's useful as a debugging aid e.g. in order to identify
SIMD32-specific codegen issues which may be masked by the SIMD32
heuristic, or cases where the heuristic is incorrectly disabling
SIMD32 shaders that offer a performance advantage.

Currently this is only enabled on Gen6+, since SIMD32 codegen support
is incomplete on earlier platforms.

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