[ARM] Make v2f64 scalar_to_vector legal

Because we mark all operations as expand for v2f64, scalar_to_vector
would end up lowering through a stack store/reload. But it is pretty
simple to implement, only inserting a D reg into an undef vector. This
helps clear up some inefficient codegen from soft calling conventions.

Differential Revision: https://reviews.llvm.org/D96153