[llvm][misexpect] Re-enable MisExpect for SampleProfiling

MisExpect was occasionally crashing under SampleProfiling, due to a division by zero.
We worked around that in D124302 by changing the assert to an early return.
This patch is intended to add a test case for the crashing scenario and
re-enable MisExpect for SampleProfiling.

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D124481