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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@54223 91177308-0d34-0410-b5e6-96231b3b80d8