cranelift: add a new resumable_trapnz instruction;

This is useful to have to allow resumable_trap to happen in loop
headers, for instance. This is the correct way to implement interrupt
checks in Spidermonkey, which are effectively resumable traps. Previous
implementation was using traps, which is wrong, since traps semantically
can't be resumed after.

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

@@ -340,7 +340,21 @@ fn define_control_flow(
                 r#"
         Trap when non-zero.
 
-        if ``c`` is zero, execution continues at the following instruction.
+        If ``c`` is zero, execution continues at the following instruction.
+        "#,
+                &formats.cond_trap,
+            )
+            .operands_in(vec![c, code])
+            .can_trap(true),
+        );
+
+        ig.push(
+            Inst::new(
+                "resumable_trapnz",
+                r#"
+        A resumable trap to be called when the passed condition is non-zero.
+
+        If ``c`` is zero, execution continues at the following instruction.
         "#,
                 &formats.cond_trap,
             )