Reduce one layer of decoding for binary operations in bitcode reading.

Removes the need to call function llvm::DecodeBinaryOp. In turn, this removes
the need for enum type llvm::Instruction::BinaryOps, llvm::Type.isFPOrFPVectorTy, and one call to llvm::convertToLLVMType.

BUG= None
R=jvoung@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/788283002