[opt] Remove support for using -O[0|1|2|3|s|z] with legacy PM in opt

When running a default pipeline (for a specific O-level) in opt it is
now expected that the new PM should be used. Only reason to use the
legacy PM is when testing a pass that is locked to the legacy PM (or
when testing single passes, for example used by the llc backend).

If a test should run both a default pipeline plus some other passes,
the solution would be to invoke opt twice (separating the default
pipeline execution from the execution of individual passes).

Starting with this patch "opt -O0" etc. will result in an error.

Differential Revision: https://reviews.llvm.org/D137663