This patch lays the groundwork needed for primop support for SIMD vectors. In
addition to the groundwork, we add support for the FloatX4# primitive type and
associated primops.

 * Add the FloatX4# primitive type and associated primops.
 * Add CodeGen support for Float vectors.
 * Compile vector operations to LLVM vector operations in the LLVM code
   generator.
 * Make the x86 native backend fail gracefully when encountering vector primops.
 * Only generate primop wrappers for vector primops when using LLVM.