Note [Unfold compulsory unfoldings in RULE LHSs]

When the user writes `RULES map coerce = coerce` as a rule, the rule
will only ever match if simpleOptExpr replaces coerce by its unfolding
on the LHS, because that is the core that the rule matching engine
will find. So do that for everything that has a compulsory
unfolding. Also see Note [Desugaring coerce as cast] in GHC.HsToCore.

However, we don't want to inline 'seq', which happens to also have a
compulsory unfolding, so we only do this unfolding only for things
that are always-active.  See Note [User-defined RULES for seq] in GHC.Types.Id.Make.