[llvm-objdump] - Split disassembleObject() into two methods. NFCI.

Currently, disassembleObject() is a ~550 lines length function.

This patch splits it into two, where first do all helper objects initializations
and calls the second which does all the rest job.
This is a straightforward split.

Differential revision: https://reviews.llvm.org/D57020

llvm-svn: 351940