Added initial experimental support for explicit vectorization.

Currently only the following platform/operations are supported:
 - SSE2 compatible architecture
 - compiler compatible with intel's SSE2 intrinsics
 - float, double and int data types
 - fixed size matrices with a storage major dimension multiple of 4 (or 2 for double)
 - scalar-matrix product, component wise: +,-,*,min,max
 - matrix-matrix product only if the left matrix is vectorizable and column major
   or the right matrix is vectorizable and row major, e.g.:
   a.transpose() * b is not vectorized with the default column major storage.
To use it you must define EIGEN_VECTORIZE and EIGEN_INTEL_PLATFORM.

Eigen/src/Core/Block.h

@@ -67,9 +67,9 @@ struct ei_traits<Block<MatrixType, BlockRows, BlockCols> >
       : (BlockRows==Dynamic ? MatrixType::MaxRowsAtCompileTime : BlockRows),
     MaxColsAtCompileTime = ColsAtCompileTime == 1 ? 1
       : (BlockCols==Dynamic ? MatrixType::MaxColsAtCompileTime : BlockCols),
-    Flags = RowsAtCompileTime == Dynamic || ColsAtCompileTime == Dynamic
+    Flags = (RowsAtCompileTime == Dynamic || ColsAtCompileTime == Dynamic
             ? (unsigned int)MatrixType::Flags
-            : (unsigned int)MatrixType::Flags &~ LargeBit,
+            : (unsigned int)MatrixType::Flags &~ LargeBit) & ~VectorizableBit,
     CoeffReadCost = MatrixType::CoeffReadCost
   };
 };