Check that POST_INC/DEC has use in DFA optimization

We'd have usually converted it into a PRE_INC if there is no use,
but that's not guaranteed. If there is no use at this point, make
sure we don't try to use the sentinel value.

Author: nikic

Zend/tests/post_inc_without_use.phpt

@@ -0,0 +1,13 @@
+--TEST--
+POST_INC without use during DFA optimization
+--FILE--
+<?php
+
+function test($n) {
+    for ($i = 0; $i < $n; !$i++) {}
+}
+
+?>
+===DONE===
+--EXPECT--
+===DONE===

ext/opcache/Optimizer/dfa_pass.c

@@ -1388,7 +1388,7 @@ void zend_dfa_optimize_op_array(zend_op_array *op_array, zend_optimizer_ctx *ctx
 					 && (ssa->var_info[result_var].type & ((MAY_BE_ANY|MAY_BE_REF|MAY_BE_UNDEF) - (MAY_BE_LONG|MAY_BE_DOUBLE))) == 0) {
 						int use = ssa->vars[result_var].use_chain;
 
-						if (op_array->opcodes[use].opcode == ZEND_IS_SMALLER
+						if (use >= 0 && op_array->opcodes[use].opcode == ZEND_IS_SMALLER
 						 && ssa->ops[use].op1_use == result_var
 						 && zend_dfa_try_to_replace_result(op_array, ssa, op_1, v)) {
 							opline->opcode = ZEND_PRE_INC;
@@ -1402,7 +1402,7 @@ void zend_dfa_optimize_op_array(zend_op_array *op_array, zend_optimizer_ctx *ctx
 					 && (ssa->var_info[result_var].type & ((MAY_BE_ANY|MAY_BE_REF|MAY_BE_UNDEF) - (MAY_BE_LONG|MAY_BE_DOUBLE))) == 0) {
 						int use = ssa->vars[result_var].use_chain;
 
-						if (op_array->opcodes[use].opcode == ZEND_IS_SMALLER
+						if (use >= 0 && op_array->opcodes[use].opcode == ZEND_IS_SMALLER
 						 && ssa->ops[use].op2_use == result_var
 						 && zend_dfa_try_to_replace_result(op_array, ssa, op_1, v)) {
 							opline->opcode = ZEND_PRE_DEC;