divb / mulb outputs to ah. Under x86-64 it's not legal to read ah if the...

divb / mulb outputs to ah. Under x86-64 it's not legal to read ah if the instruction requires a rex prefix (i.e. outputs to r8b, etc.). So issue shift right by 8 on AX and then truncate it to 8 bits instead.

llvm-svn: 40972