[NEON] Define fp16 vld and vst intrinsics conditionally

[NEON] Define fp16 vld and vst intrinsics conditionally

This patch fixes definitions of vld and vst NEON intrinsics so
that we only define them if half-precision arithmetic is
supported on the target platform, as prescribed in ACLE 2.0.

Differential Revision: https://reviews.llvm.org/D49075

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@340140 91177308-0d34-0410-b5e6-96231b3b80d8