The type synonym expander was doing redundant work by looking at same
types again and again. This patch fixes the loop code when both of the
types can be expanded, to do `O(min(n, m))` comparisons and `O(n + m)`
expansions, where `n` is expansions of the first type and `m` is
expansions of the second type.

Reported by sjcjoosten in T10547.

Test Plan:
Added a regression test that was taking several minutes to type check
before this patch.

Reviewers: bgamari, simonpj, austin, ezyang

Reviewed By: bgamari, simonpj, austin, ezyang

Subscribers: simonpj, thomie

Differential Revision: https://phabricator.haskell.org/D2198

GHC Trac Issues: #10547