Fix -data-list-register-names/-data-disassemble, minor fix in -data-list-register-values (MI)

Summary:
* Fix -data-list-register-names command: previously it ignored regno arguments and always showed all registers
* Add 'size' field to -data-disassemble command: now we are able to get an instruction's size
* Minor fix in -data-list-register-value: fix comments/code style
* Enable all tests in MiDataTestCase
* Fix the GetRegister function that gets an register by its index

These changes were tested on OS X; all MiDataTestCase tests were passed.

Reviewers: clayborg, abidh

Reviewed By: clayborg, abidh

Subscribers: clayborg, abidh, lldb-commits

Differential Revision: http://reviews.llvm.org/D7442

llvm-svn: 228393