r300/compiler: Use zero as the register index for unused sources

This fixes an invalid "Too many hardware temporaries used" error in the
case where a source reads from a temporary register with an index greater
than max_temp_regs and then the source is marked as unused before the
register allocation pass.

NOTE: This is a candidate for the 7.9 branch.

src/mesa/drivers/dri/r300/compiler/r300_fragprog_emit.c

@@ -76,6 +76,9 @@ static void use_temporary(struct r300_fragment_program_code *code, unsigned int
 
 static unsigned int use_source(struct r300_fragment_program_code* code, struct rc_pair_instruction_source src)
 {
+	if (!src.Used)
+		return 0;
+
 	if (src.File == RC_FILE_CONSTANT) {
 		return src.Index | (1 << 5);
 	} else if (src.File == RC_FILE_TEMPORARY) {

src/mesa/drivers/dri/r300/compiler/r500_fragprog_emit.c

@@ -200,6 +200,9 @@ static void use_temporary(struct r500_fragment_program_code* code, unsigned int
 
 static unsigned int use_source(struct r500_fragment_program_code* code, struct rc_pair_instruction_source src)
 {
+	if (!src.Used)
+		return 0;
+
 	if (src.File == RC_FILE_CONSTANT) {
 		return src.Index | 0x100;
 	} else if (src.File == RC_FILE_TEMPORARY) {