[x86] Infer disassembler mode from SubtargetInfo feature bits

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199649 91177308-0d34-0410-b5e6-96231b3b80d8