swr/rast: Use llvm intrinsic masked gather

swr/rast: Use llvm intrinsic masked gather

Use llvm intrinsic masked.gather instead of manual unroll for the cases
where we have vector of pointers.  Improves llvm IR debug experience by
reducing a ton of IR to a single intrinsic call. Also seems to reduce
overall stack use considerably.

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