For unsigned 8-bit division. Use movzbw to set the lower 8 bits of AX while

For unsigned 8-bit division. Use movzbw to set the lower 8 bits of AX while
clearing the upper 8-bits instead of issuing two instructions. This also
eliminates the need to target the AH register which can be problematic on
x86-64.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@31832 91177308-0d34-0410-b5e6-96231b3b80d8