the size of constant pool should be constant_pool_offset to safepoint_offset

Bug: 
Change-Id: I2290ed49ccbeaa12706f2e7c37b72c96bec11fd3
Reviewed-on: https://chromium-review.googlesource.com/721960
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48702}