[GlobalISel][AArch64] Add isel support for FP16 vector @llvm.ceil

This patch adds support for vector @llvm.ceil intrinsics when full 16 bit
floating point support isn't available.

To do this, this patch...

- Implements basic isel for G_UNMERGE_VALUES
- Teaches the legalizer about 16 bit floats
- Teaches AArch64RegisterBankInfo to respect floating point registers on
  G_BUILD_VECTOR and G_UNMERGE_VALUES
- Teaches selectCopy about 16-bit floating point vectors

It also adds

- A legalizer test for the 16-bit vector ceil which verifies that we create a
  G_UNMERGE_VALUES and G_BUILD_VECTOR when full fp16 isn't supported
- An instruction selection test which makes sure we lower to G_FCEIL when
  full fp16 is supported
- A test for selecting G_UNMERGE_VALUES

And also updates arm64-vfloatintrinsics.ll to show that the new ceiling types
work as expected.

https://reviews.llvm.org/D56682

llvm-svn: 352113