CoreTidy: Don't seq unfoldings
CoreTidy: Don't seq unfoldings

Previously we would force uf_is_value and friends to ensure that we didn't
retain a reference to the pre-tidying template, resulting in a space leak.
Instead, we now just reinitialize these fields (despite the fact that they
should not have changed). This may result in a bit more computation, but most of
the time we won't ever evaluate them anyways, so the damage shouldn't be so bad.

See #13564.

sdiehl/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc