SIMD primops are now generated using schemas that are polymorphic in
width and element type. SIMD primops are now polymorphic in vector size and element type, but only internally to the compiler. More specifically, utils/genprimopcode has been extended so that it "knows" about SIMD vectors. This allows us to, for example, write a single definition for the "add two vectors" primop in primops.txt.pp and have it instantiated at many vector types. This generates a primop in GHC.Prim for each vector type at which "add two vectors" is instantiated, but only one data constructor for the PrimOp data type, so the code generator is much, much simpler.