DeepCompile: Fix IPG bucket clearing (#7610)
PR #6993 replaces the flat IPG buffers with a dict maintaining
type-indexed buckets. The member is also renamed from
`_ipg_bucket_flat_buffer` to `ipg_buckets`.
Update the bucket clearing logic in `init_z3` accordingly.
Signed-off-by: Junjie Mao <junjie.mao@linux.alibaba.com>
