Tighten known bits for ctpop based on zero input bits

This is a cleaned up patch from the one written by John Regehr based on the findings of the Souper superoptimizer.

The basic idea here is that input bits that are known zero reduce the maximum count that the intrinsic could return. We know that the number of bits required to represent a particular count is at most log2(N)+1.

Differential Revision: http://reviews.llvm.org/D13253

llvm-svn: 250338