[AArch64] Add nvcast patterns for v4f16 and v8f16

Summary:
Constant stores of f16 vectors can create NvCast nodes from various
operand types to v4f16 or v8f16 depending on patterns in the stored
constants.  This patch adds nvcast rules with v4f16 and v8f16 values.

AArchISelLowering::LowerBUILD_VECTOR has the details on which constant
patterns generate the nvcast nodes.

Reviewers: jmolloy, srhines, ab

Subscribers: rengolin, aemerson, llvm-commits

Differential Revision: http://reviews.llvm.org/D9201

llvm-svn: 235610