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/lower.rs

@@ -212,9 +212,14 @@ pub(crate) fn input_to_reg<C: LowerCtx<I = Inst>>(
     let from_bits = ty_bits(ty) as u8;
     let inputs = ctx.get_input(input.insn, input.input);
     let in_reg = if let Some(c) = inputs.constant {
+        let masked = if from_bits < 64 {
+            c & ((1u64 << from_bits) - 1)
+        } else {
+            c
+        };
         // Generate constants fresh at each use to minimize long-range register pressure.
         let to_reg = ctx.alloc_tmp(Inst::rc_for_type(ty).unwrap(), ty);
-        for inst in Inst::gen_constant(to_reg, c, ty).into_iter() {
+        for inst in Inst::gen_constant(to_reg, masked, ty).into_iter() {
             ctx.emit(inst);
         }
         to_reg.to_reg()