Fix COPY_TMP live range construction after optimization

If we optimize the FREE away, we should switch to constructing
a normal live range, rather than a split live range.

Fixes oss-fuzz #39548.

Author: nikic

Zend/tests/coalesce_assign_optimization.phpt

@@ -0,0 +1,17 @@
+--TEST--
+Live range construction should not break if colesce assign branch is optimized away
+--FILE--
+<?php
+function test() {
+    $a[X] ??= Y;
+    var_dump($a);
+}
+define('X', 1);
+define('Y', 2);
+test();
+?>
+--EXPECT--
+array(1) {
+  [1]=>
+  int(2)
+}

Zend/zend_opcode.c

@@ -721,17 +721,22 @@ static void emit_live_range(
 			 * "null" branch, and another from the start of the "non-null" branch to the
 			 * FREE opcode. */
 			uint32_t rt_var_num = EX_NUM_TO_VAR(op_array->last_var + var_num);
-			zend_op *block_start_op = use_opline;
-
 			if (needs_live_range && !needs_live_range(op_array, orig_def_opline)) {
 				return;
 			}
 
+			kind = ZEND_LIVE_TMPVAR;
+			if (use_opline->opcode != ZEND_FREE) {
+				/* This can happen if one branch of the coalesce has been optimized away.
+				 * In this case we should emit a normal live-range instead. */
+				break;
+			}
+
+			zend_op *block_start_op = use_opline;
 			while ((block_start_op-1)->opcode == ZEND_FREE) {
 				block_start_op--;
 			}
 
-			kind = ZEND_LIVE_TMPVAR;
 			start = block_start_op - op_array->opcodes;
 			if (start != end) {
 				emit_live_range_raw(op_array, var_num, kind, start, end);