Richard Eisenberg authored Apr 10, 2018 and Ben Gamari committed Apr 10, 2018

Trac #14735 (derived from Trac #11735) found that 75% of compile
time was being spent in simplCast. This patch is the first in a series
to deal with that problem.

This particular patch actually has very little effect on performance; it
just refactors simplCast so that it builds Refl coercions less often.
Refl coercions require us to compute the type to put inside them, and
even if that's done lazily it is still work and code. Instead we use
Maybe Coercion with Nothing for Refl. This change also percolates to
pushCoTyArg and pushValArg.

Reviewers: goldfire, bgamari, simonpj

Reviewed By: simonpj

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #14737

Differential Revision: https://phabricator.haskell.org/D4395