This is a pre-requisite for implementing count-{leading,trailing}-zero
prim-ops (re #9340) and may be useful to NCG to help turn some code into
branch-less code sequences.

Test Plan: Compiles and validates in combination with clz/ctz primop impl

Reviewers: ezyang, rwbarton, simonmar, austin

Subscribers: simonmar, relrod, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D141