[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@305127 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit dedc24f891c1b93171865ed5e9c3a75d82a600c7)

Change-Id: Id45a99255124b518f221b6ad8016198277b923cb
2 files changed