[AArch64] Fix DAG selection for cmps for fp16 type

Summary: When emitting comparison for fp16, in addition to promote the LHS and RHS to fp32, we need to change the VT as well.

Reviewers: t.p.northover

Subscribers: t.p.northover, aemerson, rengolin, llvm-commits

Differential Revision: http://reviews.llvm.org/D19922

llvm-svn: 269151