[AArch64] Vector FCOPYSIGN supports Custom-lowering: mark it as such.

There's a bunch of code in LowerFCOPYSIGN that does smart lowering, and
is actually already vector-aware; let's use it instead of scalarizing!

The only interesting change is that for v2f32, we previously always used
use v4i32 as the integer vector type.
Use v2i32 instead, and mark FCOPYSIGN as Custom.

llvm-svn: 243926