This should bring signed div/rem analysis up to the same level as unsigned. We use icmp simplification to determine when the divisor is known greater than the dividend.

It's possible I've been staring at this too long and made it more complicated than necessary.

Here are attempts to prove correctness in Alive:

http://rise4fun.com/Alive/TRQp

http://rise4fun.com/Alive/DMs

Each positive test is followed by a negative test to show that we're not overstepping the boundaries of the known bits.