ISLE: Allow emitting safepoint insns

Change the implementation of emitted_insts in IsleContext from
a plain vector of instructions into a vector of tuples, where
the second element is a boolean that indicates whether this
instruction should be emitted as a safepoint.

This allows targets to emit safepoint insns via ISLE.

cranelift/codegen/src/isa/x64/lower/isle.rs

@@ -227,7 +227,7 @@ where
 
     fn emit(&mut self, inst: &MInst) -> Unit {
         for inst in inst.clone().mov_mitosis() {
-            self.emitted_insts.push(inst);
+            self.emitted_insts.push((inst, false));
         }
     }