nak: fix clippy::needless_return warnings

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27234>

src/nouveau/compiler/nak/opt_jump_thread.rs

@@ -86,7 +86,7 @@ fn jump_thread(func: &mut Function) -> bool {
         rewrite_cfg(func);
     }
 
-    return progress;
+    progress
 }
 
 fn rewrite_cfg(func: &mut Function) {

src/nouveau/compiler/nak/opt_lop.rs

@@ -22,7 +22,7 @@ fn src_as_bool(src: &Src) -> Option<bool> {
     match src.src_ref {
         SrcRef::Zero | SrcRef::False | SrcRef::Imm32(0) => Some(false),
         SrcRef::True | SrcRef::Imm32(u32::MAX) => Some(true),
-        _ => return None,
+        _ => None,
     }
 }