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Make RULES more robust in GHC.Float

Simon Peyton Jones requested to merge wip/T19582 into master

The RULES that use hand-written specialised code for overloaded class methods like floor, ceiling, truncate etc were fragile to certain transformations. This patch makes them robust. See #19582 (closed).

It's all described in Note [Rules for overloaded class methods].

No test case because currently we don't do the transformation (floating out over-saturated applications) that makes this patch have an effect. But we may so so in future, and this patch makes the RULES much more robust.

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