Specialize isnull

It can sometimes turn into a constant integer, which may lead to some
branch elimination.

src/spesh/optimize.c

@@ -80,6 +80,24 @@ MVMint16 MVM_spesh_add_spesh_slot(MVMThreadContext *tc, MVMSpeshGraph *g, MVMCol
     return g->num_spesh_slots++;
 }
 
+/* If an `isnull` is on something we know the type of or value of, then we
+ * can quickly verify the type is not based on the null REPR and turn it
+ * into a constant. */
+static void optimize_isnull(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshBB *bb,
+                            MVMSpeshIns *ins) {
+    MVMSpeshFacts *obj_facts = MVM_spesh_get_facts(tc, g, ins->operands[1]);
+    if (obj_facts->flags & MVM_SPESH_FACT_KNOWN_TYPE) {
+        MVMint32 is_null = REPR(obj_facts->type)->ID == MVM_REPR_ID_MVMNull;
+        MVMSpeshFacts *result_facts = MVM_spesh_get_facts(tc, g, ins->operands[0]);
+        MVM_spesh_use_facts(tc, g, obj_facts);
+        ins->info = MVM_op_get_op(MVM_OP_const_i64_16);
+        ins->operands[1].lit_i16 = is_null;
+        result_facts->flags |= MVM_SPESH_FACT_KNOWN_VALUE;
+        result_facts->value.i = is_null;
+        MVM_spesh_facts_depend(tc, g, result_facts, obj_facts);
+    }
+}
+
 static void optimize_repr_op(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshBB *bb,
                              MVMSpeshIns *ins, MVMint32 type_operand);
 
@@ -2066,6 +2084,9 @@ static void optimize_bb_switch(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshB
         case MVM_OP_set:
             copy_facts(tc, g, ins->operands[0], ins->operands[1]);
             break;
+        case MVM_OP_isnull:
+            optimize_isnull(tc, g, bb, ins);
+            break;
         case MVM_OP_istrue:
         case MVM_OP_isfalse:
             optimize_istrue_isfalse(tc, g, bb, ins);