expand yield to if-statements and loops

compiler/ast/ast.odin

@@ -149,6 +149,10 @@ Stmt :: struct {
 	span:       source.Span,
 	name:       symbol.Id,
 	index_name: symbol.Id,
+	// `For`/`While` loop bodies may carry a label (`blk: { ... }`); a `Yield`
+	// may target one (`yield :blk x`), letting a yield reach past an enclosing
+	// `if` to exit the labeled loop. `INVALID` when absent.
+	label:      symbol.Id,
 	type:       Type_Syntax,
 	immutable:  bool,
 	pointer_capture: bool,