[ValueTracking] don't crash when assumptions conflict (PR36270)

The last assume in the test says that %B12 is 0. 
The first assume says that %and1 is less than %B12. 
Therefore, %and1 is unsigned less than 0...does not compute.

That means this line:
Known.Zero.setHighBits(RHSKnown.countMinLeadingZeros() + 1);
...tries to set more bits than exist.

Differential Revision: https://reviews.llvm.org/D43052

llvm-svn: 324610