rdar://12100355 (part 1)

rdar://12100355 (part 1)

This revision attempts to recognize following population-count pattern:

 while(a) { c++; ... ; a &= a - 1; ... },
  where <c> and <a>could be used multiple times in the loop body.

 TODO: On X8664 and ARM, __buildin_ctpop() are not expanded to a efficent
instruction sequence, which need to be improved in the following commits.

Reviewed by Nadav, really appreciate!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@168931 91177308-0d34-0410-b5e6-96231b3b80d8