[ARM NEON] Avoid duplicated decarations
The declaration of arm neon intrinsics that are
"big endian safe" print the same code for big
and small endian targets.
This patch avoids duplicates by checking if an
intrinsic is safe to have a single definition.
(decreases header 11k lines out of 73k).
Reviewers: t.p.northover, ostannard, labrinea
Reviewed By: ostannard
Subscribers: kristof.beyls, cfe-commits, olista01
Differential Revision: https://reviews.llvm.org/D66588