[X86] Properly check that EAX is dead when copying EFLAGS.
This fixes a bug introduced in r267623, where we got smarter and avoided to save
EAX before using it. However, we failed to check if any of the subregister of
EAX were alive and thus, missed cases where we have to save EAX before using it.
The problem may happen on every X86/i386/... platform.
This fixes llvm.org/PR27624