[x86] Use 16-bit addressing where possible in 16-bit mode

[x86] Use 16-bit addressing where possible in 16-bit mode

Where "where possible" means that it's an immediate value and it's below
0x10000. In fact GAS will either truncate or error with larger values,
and will insist on using the addr32 prefix to get 32-bit addressing. So
perhaps we should do that, in a later patch.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198758 91177308-0d34-0410-b5e6-96231b3b80d8