This patch resolves suboptimal code generation reported by https://github.com/llvm/llvm-project/issues/60571 .
DAGCombiner currently converts `(x or/xor const) + y` to `(x + y) + const` if this is valid.
However, if `.. + const` is broken down into a sequences of adds with carries, the benefit is not clear, introducing two more add(-with-carry) ops (total 6) in the case of the reported issue whereas the optimal sequence must only have 4 add(-with-carry)s.
This patch resolves this issue by allowing this conversion only when (1) `.. + const` is legal or promotable, or (2) `const` is a sign bit because it does not introduce more adds.
Reviewed By: RKSimon
Differential Revision: https://reviews.llvm.org/D144116
