machinst: add a linear-scan checked variant too;

cranelift/codegen/meta/src/shared/settings.rs

@@ -25,11 +25,14 @@ pub(crate) fn define() -> SettingGroup {
         - `experimental_linear_scan` is an experimental linear scan allocator. It may take less
         time to allocate registers, but generated code's quality may be inferior. As of
         2020-04-17, it is still experimental and it should not be used in production settings.
+        - `experimental_linear_scan_checked` is the linear scan allocator with additional self
+        checks that may take some time to run, and thus these checks are disabled by default.
     "#,
         vec![
             "backtracking",
             "backtracking_checked",
             "experimental_linear_scan",
+            "experimental_linear_scan_checked",
         ],
     );