[ARM] Add ARMv8.2-A FP16 vector intrinsic

Putting back the code in commit r327189 that was reverted in r322737. The code is being committed in three stages and this one is the last stage: 1) r327455 fp16 feature flags, 2) r327836 pass half type or i16 based on FullFP16, and 3) the code here which the front-end fp16 vector intrinsic for ARM.

Differential revision https://reviews.llvm.org/D43650

llvm-svn: 328277