Simplify SDBM expressions more aggressively in operators and conversions

Extend SDBM simplification patterns to support more cases where the addition of
two expressions each involving one or two variables would result in a sum
expression that only contains one variable and thus remains in the SDBM domain.
This is made possible by the new canonical structure of SDBM where the constant
term appears once.  This simplification will be necessary to support
round-tripping of stripe expressions containing constant terms on the LHS
through affine expressions.

PiperOrigin-RevId: 269757732