Introducing "vectorized" byte on unpacket_traits structs

This is a preparation to a change on gebp_traits, where a new template
argument will be introduced to dictate the packet size, so it won't be
bound to the current/max packet size only anymore.

By having packet types defined early on gebp_traits, one has now to
act on packet types, not scalars anymore, for the enum values defined
on that class. One approach for reaching the vectorizable/size
properties one needs there could be getting the packet's scalar again
with unpacket_traits<>, then the size/Vectorizable enum entries from
packet_traits<>. It turns out guards like "#ifndef
EIGEN_VECTORIZE_AVX512" at AVX/PacketMath.h will hide smaller packet
variations of packet_traits<> for some types (and it makes sense to
keep that). In other words, one can't go back to the scalar and create
a new PacketType, as this will always lead to the maximum packet type
for the architecture.

The less costly/invasive solution for that, thus, is to add the
vectorizable info on every unpacket_traits struct as well.

Eigen/src/Core/arch/MSA/PacketMath.h

@@ -117,14 +117,14 @@ struct packet_traits<int32_t> : default_packet_traits {
 template <>
 struct unpacket_traits<Packet4f> {
   typedef float type;
-  enum { size = 4, alignment = Aligned16 };
+  enum { size = 4, alignment = Aligned16, vectorizable=true };
   typedef Packet4f half;
 };
 
 template <>
 struct unpacket_traits<Packet4i> {
   typedef int32_t type;
-  enum { size = 4, alignment = Aligned16 };
+  enum { size = 4, alignment = Aligned16, vectorizable=true };
   typedef Packet4i half;
 };
 
@@ -925,7 +925,7 @@ struct packet_traits<double> : default_packet_traits {
 template <>
 struct unpacket_traits<Packet2d> {
   typedef double type;
-  enum { size = 2, alignment = Aligned16 };
+  enum { size = 2, alignment = Aligned16, vectorizable=true };
   typedef Packet2d half;
 };