Overhaul the SDBM expression kind hierarchy

Swap the allowed nesting of sum and diff expressions: now a diff expression can
contain a sum expression, but only on the left hand side.  A difference of two
expressions sum must be canonicalized by grouping their constant terms in a
single expression.  This change of sturcture became possible thanks to the
introduction of the "direct" super-kind.  It is necessary to enable support of
sum expressions on the left hand side of the stripe expression.

SDBM expressions are now grouped into the following structure
- expression
  - varying
    - direct
      - sum <- (term, constant)
      - term
        - symbol
        - dimension
        - stripe <- (term, constant)
    - negation <- (direct)
    - difference <- (direct, term)
  - constant
The notation <- (...) denotes the types of subexpressions a compound
expression can combine.

PiperOrigin-RevId: 269337222