glsl2: Add pass for supporting variable vector indexing in rvalues.

The Mesa IR needs this to support vector indexing correctly, and
hardware backends such as 915 would want this behavior as well.

Fixes glsl-vs-vec4-indexing-2.

src/mesa/shader/ir_to_mesa.cpp

@@ -1821,7 +1821,13 @@ _mesa_glsl_compile_shader(GLcontext *ctx, struct gl_shader *shader)
 	 progress = do_dead_code_unlinked(state, shader->ir) || progress;
 	 progress = do_constant_variable_unlinked(shader->ir) || progress;
 	 progress = do_constant_folding(shader->ir) || progress;
+
 	 progress = do_vec_index_to_swizzle(shader->ir) || progress;
+	 /* Do this one after the previous to let the easier pass handle
+	  * constant vector indexing.
+	  */
+	 progress = do_vec_index_to_cond_assign(shader->ir) || progress;
+
 	 progress = do_swizzle_swizzle(shader->ir) || progress;
       } while (progress);
    }