Tobias Dammers authored Apr 20, 2018 and Ben Gamari committed Apr 20, 2018

While addressing nonlinear behavior related to coercion roles,
particularly `NthCo`, we noticed that coercion roles are recalculated
often even though they should be readily at hand already in most cases.
This patch adds a `Role` to the `NthCo` constructor so that we can cache
them rather than having to recalculate them on the fly.
https://ghc.haskell.org/trac/ghc/ticket/11735#comment:23 explains the
approach.

Performance improvement over GHC HEAD, when compiling Grammar.hs (see below):

GHC 8.2.1:
```
ghc Grammar.hs  176.27s user 0.23s system 99% cpu 2:56.81 total
```

before patch (but with other optimizations applied):
```
ghc Grammar.hs -fforce-recomp  175.77s user 0.19s system 100% cpu 2:55.78 total
```

after:
```
../../ghc/inplace/bin/ghc-stage2 Grammar.hs  10.32s user 0.17s system 98% cpu 10.678 total
```

Introduces the following regressions:

- perf/compiler/parsing001 (possibly false positive)
- perf/compiler/T9872
- perf/compiler/haddock.base

Reviewers: goldfire, bgamari, simonpj

Reviewed By: simonpj

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #11735

Differential Revision: https://phabricator.haskell.org/D4394