In the 6502, write instructions with certain indexed addressing modes always
incur the page-crossing penalty cycle, even if they don't cross a page boundary.
This was simple to implement, we just added a clause to the CASE statement that
calculates the minimum cycle count.

In the 65C02, certain write instructions were excluded from this special case.
Again this was simple to implement, we just added a new clause to the CASE
statement that guaranteed the default for these instructions.

Then we switched from the "cycles and force_cycles" model to "base_cycles and
extra_cycles". This was more complex to implement, since some special cases
needed to be in only one or only the other or both. I actually got this wrong
the first time and deleted the exception for certain write instructions from the
extra_cycles CASE expression, then had to put it back.

So now, to keep things simple, we implement this special logic in one place,
with one comment explaining what it does and why.