glsl: Specify framebuffer fetch coherency mode in lower_blend_equation_advanced().

This requires passing an extra argument to the lowering pass because
the KHR_blend_equation_advanced specification doesn't seem to define
any mechanism for the implementation to determine at compile-time
whether coherent blending can ever be used (not even an "#extension
KHR_blend_equation_advanced_coherent" directive seems to be required
in the shader source AFAICT).

In the long run we'll probably want to do state-dependent recompiles
based on the value of ctx->Color.BlendCoherent, but right now there
would be no benefit from that because the only driver that supports
coherent framebuffer fetch is i965 on SKL+ hardware, which are unable
to support the non-coherent path for the moment because of texture
layout issues, so framebuffer fetch coherency is always enabled for
them.

Reviewed-by: Plamena Manolova <plamena.manolova@intel.com>

src/compiler/glsl/ir_optimization.h

@@ -166,7 +166,7 @@ bool lower_tess_level(gl_linked_shader *shader);
 
 bool lower_vertex_id(gl_linked_shader *shader);
 bool lower_cs_derived(gl_linked_shader *shader);
-bool lower_blend_equation_advanced(gl_linked_shader *shader);
+bool lower_blend_equation_advanced(gl_linked_shader *shader, bool coherent);
 
 bool lower_subroutine(exec_list *instructions, struct _mesa_glsl_parse_state *state);
 void propagate_invariance(exec_list *instructions);