Support failed JIT test case: assign_022.phpt

Major changes are made to support statement "$a[0] = $unref", where
opcode ASSIGN_DIM is involved.

Besides, one bug in macro GC_DELREF is fixed. The reference count would
be further checked after decreasing in macro ZVAL_PTR_DTOR, hence,
instruction "subs" should be used to set the flags. After fixing this
bug, external function zend_jit_array_free() is used as the dtor for the
array "$a".

Author: shqking