[DOXYGEN] Fix doxygen and content issues in emmintrin.h

- Fixed innaccurate instruction mappings for various intrinsics.
- Fixed description of NaN handling in comparison intrinsics.
- Unify description of _mm_store_pd1 to match _mm_store1_pd.
- Fix incorrect wording in various intrinsic descriptions. Previously the descriptions used "low-order" and "high-order" when the intended meaning was "even-indexed" and "odd-indexed".
- Fix typos.
- Add missing italics command (\a) for params and fixed some parameter spellings.

This patch was made by Craig Flores

Differential Revision: https://reviews.llvm.org/D41516

llvm-svn: 321669