set constant_pool_ to proper value before trying to print it

Change-Id: Iee0da126dd3641f40c1d1847e7f1ef5d6e3e58fd
Reviewed-on: https://chromium-review.googlesource.com/916890Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#51292}