Remove pset, replace with ploadu.

We can't make guarantees on alignment for existing calls to `pset`,
so we should default to loading unaligned.  But in that case, we should
just use `ploadu` directly. For loading constants, this load should hopefully
get optimized away.

This is causing segfaults in Google Maps.

Eigen/src/Core/arch/Default/GenericPacketMathFunctionsFwd.h

@@ -17,10 +17,6 @@ namespace internal {
 // implemented in GenericPacketMathFunctions.h
 // This is needed to workaround a circular dependency.
 
-/** \internal \returns a packet with constant coefficients \a a, e.g.: (a[N-1],...,a[0]) */
-template<typename Packet, int N> EIGEN_DEVICE_FUNC inline Packet
-pset(const typename unpacket_traits<Packet>::type (&a)[N] /* a */);
-
 /***************************************************************************
  * Some generic implementations to be used by implementors
 ***************************************************************************/