[lldb] Break up CommandObjectDisassemble::DoExecute

The function consisted of a complicated set of conditions to compute the
address ranges which are to be disassembled (depending on the mode
selected by command line switches). This patch creates a separate
function for each mode, so that DoExecute is only left with the task of
figuring out how to dump the relevant ranges.

This is NFC-ish, except for one change in the error message, which is
actually an improvement.