[InstSimplify] fold sdiv/srem based on compare of dividend and divisor
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.
Each positive test is followed by a negative test to show that we're not overstepping the boundaries of the known bits.
There are extra tests for the signed-min-value special cases.
Differential Revision: https://reviews.llvm.org/D37713