Avoid an assertion failure in abstractFloats

The function GHC.Core.Opt.Simplify.Utils.abstractFloats
was carelessly calling lookupIdSubst_maybe on a CoVar;
but a precondition of the latter is being given an Id.

In fact it's harmless to call it on a CoVar, but still, the
precondition on lookupIdSubst_maybe makes sense, so I added
a test for CoVars.

This avoids a crash in a DEBUG compiler, but otherwise has
no effect. Fixes #23426.