x86: revert disassembler parts of "x86: Allow 16-bit register source for LAR and LSL"

This reverts the disassembler parts of 859aa2c8 ("x86: Allow 16-bit
register source for LAR and LSL"), adjusting testcases as necessary.
That change was itself a partial revert of c9f5b96b ("x86: correct
handling of LAR and LSL"), without actually saying so. While the earlier
commit was properly agreed upon, the partial revert was not, and hence
should not have been committed. This is even more so that the revert
part of that change wasn't even necessary to address PR gas/29844.