Guard rewritableTyVarsOfType

Authored by simonpj on Feb 9 2017, 6:12 AM.

Description

Guard rewritableTyVarsOfType

We only want to use rewriteableTyVarsOfType on CDictCan,
CFunEqCan (and maybe CIrredCan). But not CTyEqCan.

But we were -- for insolubles. So I narrowed the scope of
the insuluble kick-out.