Add -disassemble support for -show-inst and -show-encode capability llvm-mc....

Add -disassemble support for -show-inst and -show-encode capability llvm-mc. Also refactor so all MC paraphernalia are created once for all uses as much as possible. 

The test change is to account for the fact that the default disassembler behaviour has changed with regards to specifying the assembly syntax to use.

llvm-svn: 154809