AssumptionCache: Disable the verifier by default, move it behind a hidden...

AssumptionCache: Disable the verifier by default, move it behind a hidden cl::opt and verify from releaseMemory().

This is a short term solution to the problem that many passes currently fail
to update the assumption cache. In the long term the verifier should not
be controllable with a flag. We should either fix all passes to correctly
update the assumption cache and enable the verifier unconditionally or
somehow arrange for the assumption list to be updated automatically by passes.

Differential Revision: https://reviews.llvm.org/D30003

llvm-svn: 295236