Use a label-branch in CheckConstPool.

This removes a dependency on the size of the pool. Whilst the size is
checked in debug mode, it is still more robust to use a label.

This should also be cherry-picked to fix 4.4.63.20. (At that time, the
size of the pool was _not_ checked, and was sometimes wrong.)

This partially reverts 879550ce, "[arm] Don't call branch_offset within
CheckConstPool." However, branch_offset now only blocks the constant
pool if it is not already blocked.

BUG=

Review URL: https://codereview.chromium.org/1227403006

Cr-Commit-Position: refs/heads/master@{#29746}