[DOXYGEN] Documentation for the newly added x86 intrinsics.

Added doxygen comments for the newly added intrinsics in avxintrin.h, namely _mm256_cvtsd_f64, _mm256_cvtsi256_si32 and _mm256_cvtss_f32 

Added doxygen comments for the new intrinsics in emmintrin.h, namely _mm_loadu_si64 and _mm_load_sd.

Explicit parameter names were added for _mm_clflush and _mm_setcsr 

The rest of the changes are editorial, removing trailing spaces at the end of the lines.

Differential Revision: https://reviews.llvm.org/D28503

llvm-svn: 291876