Refactor UnaryOperator class

The UnaryOperator class was originally placed in llvm/IR/Instructions.h, with the other UnaryInstructions. However, I'm now thinking that it makes more sense for it to live in llvm/IR/InstrTypes.h, with BinaryOperator. It is more similar to BinaryOperator than any of the other UnaryInstructions.

NFCI

Differential Revision: https://reviews.llvm.org/D61614

llvm-svn: 360103