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.

cranelift/codegen/src/value_label.rs

@@ -91,6 +91,11 @@ pub fn build_value_labels_ranges<T>(
 where
     T: From<SourceLoc> + Deref<Target = SourceLoc> + Ord + Copy,
 {
+    // FIXME(#1523): New-style backend does not yet have debug info.
+    if isa.get_mach_backend().is_some() {
+        return HashMap::new();
+    }
+
     let values_labels = build_value_labels_index::<T>(func);
 
     let mut blocks = func.layout.blocks().collect::<Vec<_>>();