swr/rast: Start refactoring of builder/packetizer.

swr/rast: Start refactoring of builder/packetizer.

Move x86 intrinsic lowering to a separate pass. Builder now instantiates
generic intrinsics for features not supported by llvm. The separate x86
lowering pass is responsible for lowering to valid x86 for the target
SIMD architecture. Currently it's a port of existing code to get it
up and running quickly. Will eventually support optimized x86 for AVX,
AVX2 and AVX512.

Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com>