Adding front-end support to several intrinsics (bit scanning, conversion and state...

Adding front-end support to several intrinsics (bit scanning, conversion and state reading intrinsics)

Adding LLVM front-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

 Furthermore, adding clang front-end support to these conversion intrinsics: _mm256_cvtsd_f64, _mm256_cvtsi256_si32 and _mm256_cvtss_f32.

 Finally, adding tests to all of the above, as well as to the state reading intrinsics _rdpmc and _rdtsc.
  Their functionality is also specified in the Intel intrinsics guide.

Commit on behalf of Omer Paparo Bivas

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@271387 91177308-0d34-0410-b5e6-96231b3b80d8