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/machinst/mod.rs

@@ -308,14 +308,10 @@ pub trait MachBackend {
     fn reg_universe(&self) -> &RealRegUniverse;
 
     /// Machine-specific condcode info needed by TargetIsa.
-    fn unsigned_add_overflow_condition(&self) -> IntCC {
-        // TODO: this is what x86 specifies. Is this right for arm64?
-        IntCC::UnsignedLessThan
-    }
+    /// Condition that will be true when an IaddIfcout overflows.
+    fn unsigned_add_overflow_condition(&self) -> IntCC;
 
     /// Machine-specific condcode info needed by TargetIsa.
-    fn unsigned_sub_overflow_condition(&self) -> IntCC {
-        // TODO: this is what x86 specifies. Is this right for arm64?
-        IntCC::UnsignedLessThan
-    }
+    /// Condition that will be true when an IsubIfcout overflows.
+    fn unsigned_sub_overflow_condition(&self) -> IntCC;
 }