More changes to fix a space leak in the simplifier (#13426)
More changes to fix a space leak in the simplifier (#13426)

Part of e13419c55 was accidentally lost during a rebase. This commit
adds the missing change, along with some more improvements
regarding where we do and don't use `seqType`.

Also include a comment about where the space leak showed up
and a Note explaining the strategy being used here.

Test Plan: harbormaster, plus local testing on DynFlags

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D3421

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