[X86] Don't zero out %eax if both %al and %ah are used

The iterator over super and sub registers doesn't include both 8-bit
registers in its list. So if both registers are used and only one of
them is live on return, then we need to make sure that the other 8-bit
register is also marked as live and not zeroed out.

Reviewed By: nickdesaulniers

Differential Revision: https://reviews.llvm.org/D139679