[ARM] Fix arm_neon.h with -flax-vector-conversions=none, part 3

It's completely impossible to check that I've actually found all the
issues, due to the use of macros in arm_neon.h, but hopefully this time
it'll take more than a few hours for someone to find another issue.

I have no idea why, but apparently there's a rule that some, but not
all, builtins which should take an fp16 vector actually take an int8
vector as an argument.  Fix this, and add test coverage.

Differential Revision: https://reviews.llvm.org/D68838

llvm-svn: 375179