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

- 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".

This patch was made by Craig Flores

Differential Revision: https://reviews.llvm.org/D41518

llvm-svn: 321670