[ARM] Support for v4f16 and v8f16 vectors

This is the groundwork for adding the Armv8.2-A FP16 vector intrinsics, which
uses v4f16 and v8f16 vector operands and return values. All the moving parts
are tested with two intrinsics, a 1-operand v8f16 and a 2-operand v4f16
intrinsic. In a follow-up patch the rest of the intrinsics and tests will be
added.

Differential Revision: https://reviews.llvm.org/D44538

llvm-svn: 327839