v8f16 is a legal type but promoting to v16f16 would result in an illegal
type.
Let's legalize these by a combination of splitting+promoting resulting
in a pair of v4f16.
Also, we were being overly cautious with different v4f16 nodes. Mark
more of them safe to promote to v4f32.
Similar to d39b4ce3ce8a3c256e01bdec2b140777a332a633
Using "eabi" or "gnueabi" for aarch64 targets is a common mistake and
warned by Clang Driver. We want to avoid them elsewhere as well. Just
use the common "aarch64" without other triple components.
This moves the legalization of G_FMA to the action builder that can handle more
types. The existing arm64-vfloatintrinsics.ll has been removed as they are
covered in other test files.
