Revert "r300/compiler: Don't try to convert RGB to Alpha in full instructions"

This reverts commit cd2857fae1.

It breaks Unigine Heaven.

src/mesa/drivers/dri/r300/compiler/radeon_pair_schedule.c

@@ -600,8 +600,7 @@ static void is_rgb_to_alpha_possible(
 	struct rc_reader_data * reader_data = userdata;
 
 	if (!can_remap(inst->U.P.RGB.Opcode)
-	    || !can_remap(inst->U.P.Alpha.Opcode)
-	    || inst->U.P.Alpha.Opcode != RC_OPCODE_NOP) {
+	    || !can_remap(inst->U.P.Alpha.Opcode)) {
 		reader_data->Abort = 1;
 		return;
 	}