Teach SimplifyDemandedUseBits that adding or subtractings 0s from every bit...

Teach SimplifyDemandedUseBits that adding or subtractings 0s from every bit below the highest demanded bit can be simplified

If we are adding/subtractings 0s below the highest demanded bit we can just use the other operand and remove the operation.

My primary motivation is observing that we can call ShrinkDemandedConstant for the add/sub and create a 0 constant, rather than removing the add completely. In the case I saw, we modified the constant on an add instruction to a 0, but the add is not put into the worklist. So we didn't revisit it until the next InstCombine iteration. This caused an IR modification to remove add and a subsequent iteration to be ran.

With this change we get bypass the add in the first iteration and prevent the second iteration from changing anything.

Differential Revision: https://reviews.llvm.org/D31120

llvm-svn: 300075