cgen: minor optimization to `for ch in "StringLiteral"` (#9240)

vlib/v/gen/c/cgen.v

@@ -1509,13 +1509,18 @@ fn (mut g Gen) for_in_stmt(node ast.ForInStmt) {
 			}
 		}
 	} else if node.kind == .string {
+		cond := if node.cond is ast.StringLiteral || node.cond is ast.StringInterLiteral {
+			ast.Expr(g.new_ctemp_var_then_gen(node.cond, table.string_type))
+		} else {
+			node.cond
+		}
 		i := if node.key_var in ['', '_'] { g.new_tmp_var() } else { node.key_var }
 		g.write('for (int $i = 0; $i < ')
-		g.expr(node.cond)
+		g.expr(cond)
 		g.writeln('.len; ++$i) {')
 		if node.val_var != '_' {
 			g.write('\tbyte ${c_name(node.val_var)} = ')
-			g.expr(node.cond)
+			g.expr(cond)
 			g.writeln('.str[$i];')
 		}
 	} else if node.kind == .struct_ {