[X86] Use MOVZX16rr8/MOVZXrm8 when extending input for i8 udivrem.

[X86] Use MOVZX16rr8/MOVZXrm8 when extending input for i8 udivrem.

We can rely on X86FixupBWInsts to turn these into MOVZX32. This
simplifies a follow up commit to use MOVSX for i8 sdivrem with
a late optimization to use CBW when register allocation works out.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@371242 91177308-0d34-0410-b5e6-96231b3b80d8