Three fixes to various SpiderMonkey-related issues:

- Properly mask constant values down to appropriate width when
  generating a constant value directly in aarch64 backend. This was a
  miscompilation introduced in the new-isel refactor. In combination
  with failure to respect NarrowValueMode, this resulted in a very
  subtle bug when an `i32` constant was used in bit-twiddling logic.

- Add support for `iadd_ifcout` in aarch64 backend as used in explicit
  heap-check mode. With this change, we no longer fail heap-related
  tests with the huge-heap-region mode disabled.

- Remove a panic that was occurring in some tests that are currently
  ignored on aarch64, by simply returning empty/default information in
  `value_label` functionality rather than touching unimplemented APIs.
  This is not a bugfix per-se, but removes confusing panic messages from
  `cargo test` output that might otherwise mislead.

tests/all/fuzzing.rs

@@ -21,7 +21,7 @@ fn instantiate_empty_module_with_memory() {
 }
 
 #[test]
-#[cfg_attr(target_arch = "aarch64", should_panic)] // FIXME(#1523)
+#[cfg_attr(target_arch = "aarch64", ignore)] // FIXME(#1523)
 fn instantiate_module_that_compiled_to_x64_has_register_32() {
     let mut config = Config::new();
     config.debug_info(true);