Reftypes part two: add support for stackmaps.

This commit adds support for generating stackmaps at safepoints to the
new backend framework and to the AArch64 backend in particular. It has
been tested to work with SpiderMonkey.

cranelift/codegen/src/inst_predicates.rs

@@ -61,3 +61,10 @@ pub fn is_constant_64bit(func: &Function, inst: Inst) -> Option<u64> {
         _ => None,
     }
 }
+
+/// Is the given instruction a safepoint (i.e., potentially causes a GC, depending on the
+/// embedding, and so requires reftyped values to be enumerated with a stackmap)?
+pub fn is_safepoint(func: &Function, inst: Inst) -> bool {
+    let op = func.dfg[inst].opcode();
+    op.is_resumable_trap() || op.is_call()
+}