[NewPM] Don't error when there's an unrecognized pass name

This currently blocks --print-before/after with a legacy PM pass, for
example when we use the new PM for the optimization pipeline but the
legacy PM for the codegen pipeline. Also in the future when the codegen
pipeline works with the new PM there will be multiple places to specify
passes, so even when everything is using the new PM, there will still be
multiple places that can accept different pass names.

Reviewed By: hoy, ychen

Differential Revision: https://reviews.llvm.org/D94283