[llvm-objdump] Simplify conditional statements (isa<...>(Obj) => Obj->isSomeFile())

Summary: Simplify some conditional statements.

Reviewers: jhenderson, MaskRay, rupprecht

Reviewed By: MaskRay, rupprecht

Subscribers: rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75899