[opt] Add -p alias for -passes

See [1] for background.

Some people have complained that `opt -passes=instcombine` is a lot more typing than `opt -instcombine`.
As a compromise that nobody has objected to in [1], allow `opt -p instcombine`.

[1] https://discourse.llvm.org/t/rfc-legacy-opt-pass-syntax-with-new-pass-manager/65863

Reviewed By: bjope, asbirlea

Differential Revision: https://reviews.llvm.org/D136616