[AArch64] Add fallback in FastISel fp16 conversions

Summary:
- Fix assertion failures on F16 to/from int types in FastISel by falling
  back to regular ISel
- Add a testcase of various conversion cases with FastISel (-O0)

Reviewers: kristof.beyls, jmolloy, SjoerdMeijer

Reviewed By: SjoerdMeijer

Subscribers: SjoerdMeijer, llvm-commits, srhines, pirama, aemerson, rengolin, javed.absar, kristof.beyls

Differential Revision: https://reviews.llvm.org/D33734

llvm-svn: 305127