Remove unsafe AssertZext after promoting result of FP_TO_FP16

Summary:
Some target lowerings of FP_TO_FP16, for instance ARM's vcvtb.f16.f32
instruction, do not guarantee that the top 16 bits are zeroed out.
Remove the unsafe AssertZext and add tests to exercise this.

Reviewers: jmolloy, sbaranga, kristof.beyls, aadg

Subscribers: llvm-commits, srhines, aemerson

Differential Revision: http://reviews.llvm.org/D18426

llvm-svn: 264285