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/isa/aarch64/mod.rs

@@ -1,5 +1,6 @@
 //! ARM 64-bit Instruction Set Architecture.
 
+use crate::ir::condcodes::IntCC;
 use crate::ir::Function;
 use crate::isa::Builder as IsaBuilder;
 use crate::machinst::{
@@ -92,6 +93,19 @@ impl MachBackend for AArch64Backend {
     fn reg_universe(&self) -> &RealRegUniverse {
         &self.reg_universe
     }
+
+    fn unsigned_add_overflow_condition(&self) -> IntCC {
+        // Unsigned `>=`; this corresponds to the carry flag set on aarch64, which happens on
+        // overflow of an add.
+        IntCC::UnsignedGreaterThanOrEqual
+    }
+
+    fn unsigned_sub_overflow_condition(&self) -> IntCC {
+        // unsigned `<`; this corresponds to the carry flag cleared on aarch64, which happens on
+        // underflow of a subtract (aarch64 follows a carry-cleared-on-borrow convention, the
+        // opposite of x86).
+        IntCC::UnsignedLessThan
+    }
 }
 
 /// Create a new `isa::Builder`.