nir: Use a single bit for the dual-source blend index

The only values allowed are 0 and 1, and the value is checked before
assigning.

This is a copy of 8eeca7a56c that seems to have been made to the glsl
ir type after it was copied for use in nir but before nir landed.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>

src/glsl/nir/nir.h

@@ -292,9 +292,13 @@ typedef struct {
       unsigned int driver_location;
 
       /**
-       * output index for dual source blending.
+       * Output index for dual source blending.
+       *
+       * \note
+       * The GLSL spec only allows the values 0 or 1 for the index in \b dual
+       * source blending.
        */
-      int index;
+      unsigned index:1;
 
       /**
        * Initial binding point for a sampler or UBO.