[llvm-stress] Remove dependency to legacy pass manager

This patch removes the dependency to the legacy pass manager when
building llvm-stress.

Instead of setting up a pass manager at all we just run verifyModule()
to check that the generated IR satisfies the verifier, and then
we use Module::print() to output the IR. No need to setup passes and
populating a pass manager since we aren't doing anything more fancy
than that.

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D134802