Summary:
The form
  case na# ==# nb# of
    0# -> ...
    _  -> ...
sometimes generates convoluted assembly, see #10676.
timesInt2Integer was the most spectacular offender, especially as
it is a rather cheap function overall (no calls to gmp).

I checked a few instances and some of the old generated assembly
was fine already, but I changed them all for consistency. The new
form is also more consistent with use of these primops in general.

Test Plan: validate

Reviewers: hvr, bgamari, goldfire, austin

Reviewed By: hvr

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1094