nir: Add blend_const_color_rgba sysval

nir: Add blend_const_color_rgba sysval

This represents a float vec4 constant color, as passed to glBlendColor.
While the existing 4 shader sysvals are retained to minimize code churn,
a single vectorized intrinsic is required for efficient blending on
vector architectures. (This may also apply to archictectures like
Bifrost where ALU is scalar but load/store is vector; it largely depends
on how blending is implemented per-driver.)

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>