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.
As a side effect, as the last use was removed, the
TargetMaching::adjustPassManager function has been removed.
I should probably add a test case for this.