R600/SI: Move i64 -> v2i32 load promotion into AMDGPUDAGToDAGISel::Select()

We used to do this promotion during DAG legalization, but this
caused an infinite loop in ExpandUnalignedLoad() because it assumed
that i64 loads were legal if i64 was a legal type.

It also seems better to report i64 loads as legal, since they actually
are and we were just promoting them to simplify our tablegen files.

llvm-svn: 226945