Reapply [SelectionDAG] Handle NSW for ADD/SUB in computeKnownBits()

This exposed a miscompile due to incorrect flag preservation in
integer type legalization, which has been fixed in D151472.

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This patch is a continuation of D150110. It separates the cases for
ADD and SUB into their own cases so that computeForAddSub can be
directly called and the NSW flag passed. This allows better
optimization when the NSW flag is enabled, and allows fixing up the
TODO that was there previously in SimplifyDemandedBits.

Differential Revision: https://reviews.llvm.org/D150769