Bitmap: Use foldl' instead of foldr
Bitmap: Use foldl' instead of foldr

These are producing StgWords so foldl' is the natural choice. I'm not
sure how I didn't notice this when I wrote D1041.

Test Plan: Validate

Reviewers: austin, simonmar

Reviewed By: simonmar

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2955

GHC Trac Issues: #7450

ezyang/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc,shlevy/ghc