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	Fix a predicate-simplification bug
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Fixes a bug pointed out by Marcin

    data R = R {f :: Int}
    foo:: (?x :: Int) => R -> R
    foo r = r {f = ?x}

    Test.hs:4:
	Could not deduce `?x :: Int' from the context ()
	arising from use of implicit parameter `?x' at Test.hs:4
	In the record update: r {f = ?x}
	In the definition of `foo': r {f = ?x}

The predicate simplifier was declining to 'inherit' an
implicit parameter.  This is right for a let-binding, but
wrong for an expression binding.  For example, a simple
expression type signature:

		(?x + 1) :: Int

This was rejected because the ?x constraint could not be
floated out -- but that's wrong for expressions.