[AArch64][GlobalISel] Actually select G_INTRINSIC_TRUNC

Apparently FileCheck wasn't actually matching the fallback check lines in
arm64-vfloatintrinsics.ll properly. So, there were selection fallbacks for
G_INTRINSIC_TRUNC there.

Actually hook it up into AArch64InstructionSelector.cpp and write a proper
selection test.

I guess I'll figure out the FileCheck magic to make the fallback checks work
properly in arm64-vfloatintrinsics.ll.

llvm-svn: 359030