Detect more trivially redundant constraints better

- detect more trivially redundant constraints in
  FlatAffineConstraints::removeTrivialRedundantConstraints. Redundancy due to
  constraints that only differ in the constant part (eg., 32i + 64j - 3 >= 0, 32 +
  64j - 8 >= 0) is now detected.  The method is still linear-time and does
  a single scan over the FlatAffineConstraints buffer. This detection is useful
  and needed to eliminate redundant constraints generated after FM elimination.

- update GCDTightenInequalities so that we also normalize by the GCD while at
  it. This way more constraints will show up as redundant (232i - 203 >= 0
  becomes i - 1 >= 0 instead of 232i - 232 >= 0) without having to call
  normalizeConstraintsByGCD.

- In FourierMotzkinEliminate, call GCDTightenInequalities and
  normalizeConstraintsByGCD before calling removeTrivialRedundantConstraints()
  - so that more redundant constraints are detected. As a result, redundancy
    due to constraints like i - 5 >= 0, i - 7 >= 0, 2i - 5 >= 0, 232i - 203 >=
    0 is now detected (here only i >= 7 is non-redundant).

As a result of these, a -memref-bound-check on the added test case runs in 16ms
instead of 1.35s (opt build) and no longer returns a conservative result.

PiperOrigin-RevId: 235983550