Make tagForCon non-linear

Computing the number of constructors for TyCon is linear
in the number of constructors.
That's wasteful if all you want to check is if that
number is smaller than what fits in tag bits
(usually 8 things).

What this change does is to use a function that can
determine the ineqaulity without computing the size.

This improves compile time on a module with a
data type that has 10k constructors.
The variance in total time is (suspiciously) high,
but going by the best of 3 the numbers are 8.186s vs 7.511s.
For 1000 constructors the difference isn't noticeable:
0.646s vs 0.624s.
The hot spots were cgDataCon and cgEnumerationTyCon
where tagForCon is called in a loop.

One alternative would be to pass down the size.

Test Plan: harbormaster

Reviewers: bgamari, simonmar, austin

Reviewed By: simonmar

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D4116