[4.5] Doloop improvement patches, 3/7

[Bernd Schmidt]

For unsigned comparisons, the two following expressions are equivalent:
 A + B < A    <=>    A + B < B

If B is a constant, this is also equivalent to
 A >= -B

The latter form is simpler and should be used if possible; this patch
teaches simplify_relational_operation_1 about it.  This helps with
certain expressions seen when trying to prove facts about doloops.


Bernd
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