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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