nak: Match on the SrcRef directly in Src::is_nonzero()

Calling as_u32() isn't really doing us any good here since we want to
fail for SrcRef::Zero anyway.  Also assert that src_mod == None since we
don't handle FNeg in this path.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/34818>

src/nouveau/compiler/nak/ir.rs

@@ -1304,7 +1304,8 @@ impl Src {
     }
 
     pub fn is_nonzero(&self) -> bool {
-        matches!(self.as_u32(), Some(x) if x != 0)
+        assert!(self.src_mod.is_none());
+        matches!(self.src_ref, SrcRef::Imm32(x) if x != 0)
     }
 
     pub fn is_fneg_zero(&self, src_type: SrcType) -> bool {