The implementation temporarily modifies jssp to avoid needing a scratch
register, then restores it afterwards. However, the exception path
wasn't properly restoring the value.

With this patch, failures in this part of AssertStackConsistency get
reported properly (with backtrace and a BailoutReason).

BUG=

Review URL: https://codereview.chromium.org/1556993004

Cr-Commit-Position: refs/heads/master@{#33135}