[DemandedBits] Improve accuracy of Add propagator

[DemandedBits] Improve accuracy of Add propagator

The current demand propagator for addition will mark all input bits at and right of the alive output bit as alive. But carry won't propagate beyond a bit for which both operands are zero (or one/zero in the case of subtraction) so a more accurate answer is possible given known bits.

I derived a propagator by working through truth tables and using a bit-reversed addition to make demand ripple to the right, but I'm not sure how to make a convincing argument for its correctness in the comments yet. Nevertheless, here's a minimal implementation and test to get feedback.

This would help in a situation where, for example, four bytes (<128) packed into an int are added with four others SIMD-style but only one of the four results is actually read.

Known A:     0_______0_______0_______0_______
Known B:     0_______0_______0_______0_______
AOut:        00000000001000000000000000000000
AB, current: 00000000001111111111111111111111
AB, patch:   00000000001111111000000000000000

Committed on behalf of: @rrika (Erika)

Differential Revision: https://reviews.llvm.org/D72423