Adding back-end support to two bit scanning intrinsics

Adding LLVM back-end support to two intrinsics dealing with bit scan: _bit_scan_forward and _bit_scan_reverse.
Their functionality is as described in Intel intrinsics guide:
https://software.intel.com/sites/landingpage/IntrinsicsGuide/#text=_bit_scan_forward&expand=371,370
https://software.intel.com/sites/landingpage/IntrinsicsGuide/#text=_bit_scan_reverse&expand=371,370

Commit on behalf of Omer Paparo Bivas


Differential Revision: http://reviews.llvm.org/D19915

llvm-svn: 271386