Reapply r54147 with a constraint to only use the 8-bit

subreg form on x86-64, to avoid the problem with x86-32
having GPRs that don't have 8-bit subregs.

Also, change several 16-bit instructions to use 
equivalent 32-bit instructions. These have a smaller
encoding and avoid partial-register updates.

llvm-svn: 54223