[opt] Directly translate -O# to -passes='default<O#>'

Right now when we see -O# we add the corresponding 'default<O#>' into
the list of passes to run when translating legacy -pass-name. This has
the side effect of not using the default AA pipeline.

Instead, treat -O# as -passes='default<O#>', but don't allow any other
-passes or -pass-name. I think we can keep `opt -O#` as shorthand for
`opt -passes='default<O#>` but disallow anything more than just -O#.

Tests need to be updated to not use `opt -O# -pass-name`.

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D112036