Prepare for making SwitchInstProfUpdateWrapper strict

This patch removes the test part that relates to the non-strict
behavior of SwitchInstProfUpdateWrapper and changes
the assertion to llvm_unreachable() to allow the check in
release builds.
This patch prepares SwitchInstProfUpdateWrapper to become
strict with one line change. That is need to revert it easily
if any failure will arise.

llvm-svn: 365439