i965: handle OPCODE_SWZ in the glsl path

glsl compiler will not generate OPCODE_SWZ, and as a first step it would
be translated away to a MOV anyway (why?), but later internally this opcode is
generated (for EXT_texture_swizzling).
(cherry picked from commit 4ef1f8e3b5)

src/mesa/drivers/dri/i965/brw_wm_glsl.c

@@ -2681,6 +2681,7 @@ static void brw_wm_emit_glsl(struct brw_context *brw, struct brw_wm_compile *c)
 		emit_trunc(c, inst);
 		break;
 	    case OPCODE_MOV:
+	    case OPCODE_SWZ:
 		emit_mov(c, inst);
 		break;
 	    case OPCODE_DP3: