Throw away an SCC on a single variable.  This seems to be justified:
there can be no work to attribute to the current cost centre when
evaluating a lone variable, other than the act of entering the
closure, and possibly returning immediately if it is a variable.

This also fixes a bug in profiling, which showed up as incorrect
transformations made by the simplifier resulting in extra strictness.
The simplifier assumes (in Simplify.simplLazyBind) that (let x = e in
x) will have been turned into x, but this isn't true if there's an SCC
around the x.