Refactor: simplify boolean expressions in llvm-objdump

Simplify boolean expressions involving `true` and `false` with `clang-tidy`.

Actually upon inspection a bunch of these boolean variables could be
factored away entirely anyway - using find_if and then testing the
result before using it. This also helps reduce indentation in the code
anyway - and a bunch of other related simplification fell out nearby so
I just committed all of that.

Patch by Richard Thomson (legalize@xmission.com)

Differential Revision: http://reviews.llvm.org/D8517

llvm-svn: 232984