[x86] fix AVX FP cmp intrinsic documentation (PR28110)

This copies the text used in the #define statements to the code comments. 
The conflicting text comes from AMD manuals, but those are wrong. Sadly, 
that FP cmp text has not been updated even after some docs were updated 
for Zen:
http://support.amd.com/en-us/search/tech-docs 
( AMD64 Architecture Programmer's Manual Volume 4 )

See PR28110 for more discussion:
https://bugs.llvm.org/show_bug.cgi?id=28110

Differential Revision: https://reviews.llvm.org/D31428

llvm-svn: 300068