sim/h8300: avoid self assignment

sim/h8300: avoid self assignment

There are two places in the h8300 simulator where we assign a variable
to itself.  Clang gives a warning for this, which is converted into an
error by -Werror.

Silence the warning by removing the self assignments.  As these
assignments were in a complex if/then/else tree, rather than try to
adjust all the conditions, I've just replaced the self assignments
with a comment and an empty statement.