Summary:
The 'popcnt r16, r/m16' instruction only writes the low 16 bits of
the destination register, so we have to zero-extend the result to
a full word as popCnt16# is supposed to return a Word#.

For popCnt8# we could instead zero-extend the input to 32 bits
and then do a 32-bit popcnt, and not have to zero-extend the result.
LLVM produces the 16-bit popcnt sequence with two zero extensions,
though, and who am I to argue?

Test Plan:
 - ran "make TEST=cgrun071 EXTRA_HC_OPTS=-msse42"
 - then ran again adding "WAY=optasm", and verified that
   the popcnt sequences we generate match the ones produced
   by LLVM for its @llvm.ctpop.* intrinsics

Reviewers: austin, hvr, tibbe

Reviewed By: austin, hvr, tibbe

Subscribers: phaskell, hvr, simonmar, relrod, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D147

GHC Trac Issues: #9435