[x86] Support i386-*-*-code16 triple for emitting 16-bit code
[x86] Infer disassembler mode from SubtargetInfo feature bits

Aside from cleaning up the code, this also adds support for the -code16
environment and actually enables the MODE_16BIT mode that was previously
not accessible.

There is no point adding any testing for 16-bit yet though; basically
nothing will work because we aren't handling the OpSize prefix correctly
for 16-bit mode.
[x86] Fix 16-bit handling of OpSize bit

When disassembling in 16-bit mode the meaning of the OpSize bit is
inverted. Instructions found in the IC_OPSIZE context will actually
*not* have the 0x66 prefix, and instructions in the IC context will
have the 0x66 prefix. Make use of the existing special-case handling
for the 0x66 prefix being in the wrong place, to cope with this.
[x86] Fix disassembly of callw instruction

Not quite sure why this was marked isAsmParserOnly, but it means that the
disassembler can't see it either.
[x86] Rename MOVSD/STOSD/LODSD/OUTSD to MOVSL/STOSL/LODSL/OUTSL

The disassembler has a special case for 'L' vs. 'W' in its heuristic for
checking for 32-bit and 16-bit equivalents. We could expand the heuristic,
but better just to be consistent in using the 'L' suffix.
[x86] Fix 16-bit disassembly of JCXZ/JECXZ
[x86] Fix disassembly of MOV16ao16 et al.

The addition of IC_OPSIZE_ADSIZE in r198759 wasn't quite complete. It
also turns out to have been unnecessary. The disassembler handles the
AdSize prefix for itself, and doesn't care about the difference between
(e.g.) MOV8ao8 and MOB8ao8_16 definitions. So just let them coexist and
don't worry about it.