Codegen: Add ref.is_invalid instruction

Spidermonkey returns a sentinel ref value of '-1' from some VM functions
to indicate failure. This commit adds an instruction analagous to ref.is_null
that checks for this value.

cranelift/codegen/meta/src/shared/instructions.rs

@@ -3190,6 +3190,24 @@ pub(crate) fn define(
         .operands_out(vec![a]),
     );
 
+    let a = &Operand::new("a", b1);
+    let x = &Operand::new("x", Ref);
+
+    ig.push(
+        Inst::new(
+            "is_invalid",
+            r#"
+        Reference verification.
+
+        The condition code determines if the reference type in question is
+        invalid or not.
+        "#,
+            &formats.unary,
+        )
+        .operands_in(vec![x])
+        .operands_out(vec![a]),
+    );
+
     let Cond = &Operand::new("Cond", &imm.intcc);
     let f = &Operand::new("f", iflags);
     let a = &Operand::new("a", b1);