Disable most replacements and use checked mem access in them

Since GetPointer logs, this is a simpler alternative to #6625, and will also correctly trigger the debugger.

This will also workaround the bugs we seem to be hitting with some replacements for now until we can figure out what's going wrong.

Unfortunately, there will be a small performance hit (won't be noticeable in all games but will affect most) due to the replacements no longer being used and the checked memory access (the latter will be very small, I think.)

Also cleans up some of the downcount semantics in replacements, which I'm pretty sure were wrong.  Still, doesn't seem like that's enough to fix the glitches.

-[Unknown]