Fix the remaining MUL8 and DIV8 to define AX instead of AL,AH.

Fix the remaining MUL8 and DIV8 to define AX instead of AL,AH.

These instructions technically define AL,AH, but a trick in X86ISelDAGToDAG
reads AX in order to avoid reading AH with a REX instruction.

Fix PR6489.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@97742 91177308-0d34-0410-b5e6-96231b3b80d8