Fix bug while building FP16 constant vectors for AArch64

Summary: Building FP16 constant vectors caused the FP16 data to be bitcast to i64.  This patch creates a BITCAST node with the correct value, and adds a test to verify correct handling.

Reviewers: mcrosier

Reviewed By: mcrosier

Subscribers: mcrosier, jmolloy, ab, srhines, llvm-commits, rengolin, aemerson

Differential Revision: http://reviews.llvm.org/D8369

llvm-svn: 232562