Add a -disable-16bit flag and associated support for experimenting with

Add a -disable-16bit flag and associated support for experimenting with
disabling the use of 16-bit operations on x86. This doesn't yet work for
inline asms with 16-bit constraints, vectors with 16-bit elements,
trampoline code, and perhaps other obscurities, but it's enough to try
some experiments.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@80930 91177308-0d34-0410-b5e6-96231b3b80d8