[Spesh] Switch order of insert

We did a insert-before-current, which we then had to fix up because
the annotations would be on the wrong operator; then we'd have to call
both the optimize_istrue_isfalse and optimize_iffy (because the
optimizer doesn't process the instructions 'behind' it). And the more
I thought about it, the sillier that really is. If we stick to the
convention that whenever we add an instruction we push it forward, we
will never have such problems, and moreover, the behaviour of the
optimizer as a whole is far more understandable.

src/spesh/optimize.c

@@ -1020,32 +1020,35 @@ static void optimize_istrue_isfalse(MVMThreadContext *tc, MVMSpeshGraph *g, MVMS
  *
  * If the optimization has yielded a known value, we may be able to remove the
  * branch entirely (e.g. optimize_iffy)
+ *
+ * We push the newly split if_i / unless_i forward (rather than pushing the
+ * istrue 'backward'), so that the 'normal' optimizer routines pick them up,
+ * rather than having to force picking them ourselves.
  */
 static void optimize_object_conditional(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshIns *ins, MVMSpeshBB *bb) {
     MVMSpeshOperand temp      = MVM_spesh_manipulate_get_temp_reg(tc, g, MVM_reg_int64);
+    MVMSpeshOperand condition = ins->operands[0];
+    MVMSpeshOperand target    = ins->operands[1];
     MVMSpeshFacts *temp_facts = MVM_spesh_get_facts(tc, g, temp);
     MVMSpeshIns *new_ins      = MVM_spesh_alloc(tc, g, sizeof( MVMSpeshIns ));
 
-    /* Create a new instruction to boolify the input operand of if_o */
-    new_ins->info = MVM_op_get_op(MVM_OP_istrue);
+    /* Insert if_i/unless_i after the current one */
+    new_ins->info = MVM_op_get_op(ins->info->opcode == MVM_OP_unless_o ? MVM_OP_unless_i : MVM_OP_if_i);
     new_ins->operands = MVM_spesh_alloc(tc, g, sizeof(MVMSpeshOperand) * 2);
     new_ins->operands[0] = temp;
-    new_ins->operands[1] = ins->operands[0];
-    MVM_spesh_manipulate_insert_ins(tc, bb, ins->prev, new_ins);
+    new_ins->operands[1] = target;
+    MVM_spesh_manipulate_insert_ins(tc, bb, ins, new_ins);
 
-    /* Tweak existing instruction to if_i / unless_i on result value */
-    ins->info = MVM_op_get_op(ins->info->opcode == MVM_OP_unless_o ? MVM_OP_unless_i : MVM_OP_if_i);
+    /* Tweak existing instruction to istrue */
+    ins->info = MVM_op_get_op(MVM_OP_istrue);
+    ins->operands[1] = condition;
     ins->operands[0] = temp;
+
     temp_facts->usages++;
     temp_facts->writer = new_ins;
 
-    /* Now that we have decomposed the if_o, annotations should be moved to
-     * the new_ins */
-    new_ins->annotations = ins->annotations;
-    ins->annotations = NULL;
-
     /* try to optimize the istrue */
-    optimize_istrue_isfalse(tc, g, bb, new_ins);
+    optimize_istrue_isfalse(tc, g, bb, ins);
 
     /* Release the temporary register */
     MVM_spesh_manipulate_release_temp_reg(tc, g, temp);