SDBM: support sum expressions on the LHS of stripe expressions

Introduce support for applying the stripe operator to sum expressions, as in
  (x + A) # B = x + A - (x + A) mod B.
This is required to represent a combination of tiling and padding in the SDBM
framework, and is a valid SDBM construct that was not originally supported.

PiperOrigin-RevId: 269758807