Add bit scan {forward,reverse} insns to x86 NCG

Authored by hvr.

Description

Add bit scan {forward,reverse} insns to x86 NCG

This is a pre-requisite for implementing count-{leading,trailing}-zero
prim-ops (re Trac #9340)

Reviewers: ezyang, rwbarton, simonmar, austin

Subscribers: simonmar, relrod, ezyang, carter

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