Use tyCoVarsOfType for CTyEqCan in shouldSplitWD

Authored by simonpj on Feb 3 2017, 10:15 AM.

Description

Use tyCoVarsOfType for CTyEqCan in shouldSplitWD

An ASSERT failure in rewritableTyVars made me realise
that there was an outright bug in shouldSplitWD. See
the long Note [Splitting WD constraints].