Save register before throwing undef var notice

Otherwise we may clobber it while throwing the undef var notice.
This makes the implementation for assign_dim_op line up with
fetch_dim.

Author: nikic

ext/opcache/jit/zend_jit_x86.dasc

@@ -6454,6 +6454,9 @@ static int zend_jit_assign_dim_op(dasm_State **Dst, const zend_op *opline, uint3
 			|	CMP_ZVAL_TYPE op1_addr, IS_FALSE
 			|	jg >7
 		}
+		if (Z_REG(op1_addr) != ZREG_FP) {
+			|	mov T1, Ra(Z_REG(op1_addr)) // save
+		}
 		if (op1_info & MAY_BE_UNDEF) {
 			if (op1_info & (MAY_BE_NULL|MAY_BE_FALSE)) {
 				|	IF_NOT_ZVAL_TYPE op1_addr, IS_UNDEF, >1
@@ -6464,9 +6467,6 @@ static int zend_jit_assign_dim_op(dasm_State **Dst, const zend_op *opline, uint3
 			|1:
 		}
 		|	// ZVAL_ARR(container, zend_new_array(8));
-		if (Z_REG(op1_addr) != ZREG_FP) {
-			|	mov T1, Ra(Z_REG(op1_addr)) // save
-		}
 		|	EXT_CALL _zend_new_array_0, r0
 		if (Z_REG(op1_addr) != ZREG_FP) {
 			|	mov Ra(Z_REG(op1_addr)), T1 // restore

ext/opcache/tests/jit/assign_dim_op_002.phpt

@@ -0,0 +1,13 @@
+--TEST--
+JIT ASSIGN_DIM_OP: Undefined variable
+--INI--
+opcache.enable=1
+opcache.enable_cli=1
+opcache.file_update_protection=0
+opcache.jit_buffer_size=1M
+--FILE--
+<?php
+$a[] &= 1;
+?>
+--EXPECTF--
+Warning: Undefined variable $a in %s on line %d