nak: Allows predicate in legalize_ext_instr

With OpLd now having a predicate, we forgot to update legalize_ext_instr
to allow predicates for it.

We should really get ride of those functions but for now let's keep it
simple and sync the implementation to what SM20 backend have.

Signed-off-by: Mary Guillemard <mary@mary.zone>
Fixes: 9d90cbc314 ("nak: add input predicate to load_global_nv and OpLd")
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/40934>

src/nouveau/compiler/nak/sm32.rs

@@ -2503,7 +2503,7 @@ fn legalize_ext_instr(op: &mut impl SrcsAsSlice, _b: &mut LegalizeBuilder) {
                 panic!("ALU srcs must be legalized explicitly");
             }
             SrcType::Pred => {
-                panic!("Predicates must be legalized explicitly");
+                assert!(src_is_reg(src, RegFile::Pred));
             }
             SrcType::Carry => {
                 panic!("Carry values must be legalized explicitly");

src/nouveau/compiler/nak/sm50.rs

@@ -449,7 +449,7 @@ fn legalize_ext_instr(op: &mut impl SrcsAsSlice, _b: &mut LegalizeBuilder) {
                 panic!("ALU srcs must be legalized explicitly");
             }
             SrcType::Pred => {
-                panic!("Predicates must be legalized explicitly");
+                assert!(src_is_reg(src, RegFile::Pred));
             }
             SrcType::Carry => {
                 panic!("Carry values must be legalized explicitly");