[CodeGen][ARM] Coerce FP16 vectors to integer vectors when needed

Summary:
On targets that do not support FP16 natively LLVM currently legalizes
vectors of FP16 values by scalarizing them and promoting to FP32. This
causes problems for the following code:

  void foo(int, ...);

  typedef __attribute__((neon_vector_type(4))) __fp16 float16x4_t;
  void bar(float16x4_t x) {
    foo(42, x);
  }

According to the AAPCS (appendix A.2) float16x4_t is a containerized
vector fundamental type, so 'foo' expects that the 4 16-bit FP values
are packed into 2 32-bit registers, but instead bar promotes them to
4 single precision values.

Since we already handle scalar FP16 values in the frontend by
bitcasting them to/from integers, this patch adds similar handling for
vector types and homogeneous FP16 vector aggregates.

One existing test required some adjustments because we now generate
more bitcasts (so the patch changes the test to target a machine with
native FP16 support).

Reviewers: eli.friedman, olista01, SjoerdMeijer, javed.absar, efriedma

Reviewed By: javed.absar, efriedma

Subscribers: efriedma, kristof.beyls, cfe-commits, chrib

Differential Revision: https://reviews.llvm.org/D50507

llvm-svn: 342034