Fix ModR/M byte output for 16-bit addressing modes (PR18220)

Add some tests to validate correct register selection, including a fix
to an existing test which was requiring the *wrong* output.

Patch from David Woodhouse.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198566 91177308-0d34-0410-b5e6-96231b3b80d8