Better error message when specializations depend on Vars or RVars
Better error message when specializations depend on Vars or RVars

Fixes #1077

psuriana/Halide,kgnk/Halide,tdenniston/Halide,ronen/Halide,tdenniston/Halide,kgnk/Halide,kgnk/Halide,psuriana/Halide,tdenniston/Halide,kgnk/Halide,ronen/Halide,ronen/Halide,ronen/Halide,kgnk/Halide,jiawen/Halide,ronen/Halide,psuriana/Halide,kgnk/Halide,tdenniston/Halide,jiawen/Halide,kgnk/Halide,jiawen/Halide,tdenniston/Halide,psuriana/Halide,psuriana/Halide,psuriana/Halide,jiawen/Halide,tdenniston/Halide,psuriana/Halide,jiawen/Halide,ronen/Halide,jiawen/Halide,ronen/Halide,tdenniston/Halide,jiawen/Halide,ronen/Halide,kgnk/Halide,tdenniston/Halide