Reorganise TcSimplify (again); FIX Trac #1919

Reorganise TcSimplify (again); FIX Trac #1919

This was a bit tricky.  We had a "given" dict like (d7:Eq a); then it got
supplied to reduceImplication, which did some zonking, and emerged with
a "needed given" (d7:Eq Int). That got everything confused.

I found a way to simplify matters significantly.  Now reduceContext
	- first deals with methods/literals/dictionaries
	- then deals with implications
Separating things in this way not only made the bug go away, but
eliminated the need for the recently-added "needed-givens" results returned
by checkLoop.  Hurrah.

It's still a swamp.  But it's a bit better.