custom_do_par_for was not respected on osx
custom_do_par_for was not respected on osx

Bug popped up while testing opencl backend
ronen/Halide,kgnk/Halide,psuriana/Halide,myrtleTree33/Halide,myrtleTree33/Halide,dan-tull/Halide,ronen/Halide,smxlong/Halide,kenkuang1213/Halide,ayanazmat/Halide,kgnk/Halide,kgnk/Halide,dan-tull/Halide,smxlong/Halide,smxlong/Halide,fengzhyuan/Halide,dan-tull/Halide,delcypher/Halide,psuriana/Halide,rodrigob/Halide,lglucin/Halide,smxlong/Halide,fengzhyuan/Halide,mcanthony/Halide,delcypher/Halide,delcypher/Halide,kenkuang1213/Halide,smxlong/Halide,rodrigob/Halide,dougkwan/Halide,mcanthony/Halide,mcanthony/Halide,rodrigob/Halide,jiawen/Halide,gchauras/Halide,damienfir/Halide,delcypher/Halide,mcanthony/Halide,fengzhyuan/Halide,adasworks/Halide,damienfir/Halide,fengzhyuan/Halide,psuriana/Halide,aam/Halide,aam/Halide,kenkuang1213/Halide,lglucin/Halide,lglucin/Halide,psuriana/Halide,psuriana/Halide,kenkuang1213/Halide,dougkwan/Halide,ayanazmat/Halide,ronen/Halide,kgnk/Halide,jiawen/Halide,smxlong/Halide,dougkwan/Halide,jiawen/Halide,kgnk/Halide,fengzhyuan/Halide,jiawen/Halide,kgnk/Halide,fengzhyuan/Halide,ronen/Halide,fengzhyuan/Halide,lglucin/Halide,kgnk/Halide,adasworks/Halide,damienfir/Halide,gchauras/Halide,ayanazmat/Halide,myrtleTree33/Halide,mcanthony/Halide,adasworks/Halide,tdenniston/Halide,jiawen/Halide,damienfir/Halide,tdenniston/Halide,adasworks/Halide,dougkwan/Halide,gchauras/Halide,adasworks/Halide,tdenniston/Halide,myrtleTree33/Halide,dougkwan/Halide,rodrigob/Halide,ronen/Halide,damienfir/Halide,delcypher/Halide,rodrigob/Halide,ronen/Halide,aam/Halide,delcypher/Halide,lglucin/Halide,delcypher/Halide,dan-tull/Halide,adasworks/Halide,lglucin/Halide,ayanazmat/Halide,ayanazmat/Halide,rodrigob/Halide,dougkwan/Halide,tdenniston/Halide,psuriana/Halide,ayanazmat/Halide,mcanthony/Halide,damienfir/Halide,psuriana/Halide,kenkuang1213/Halide,jiawen/Halide,kenkuang1213/Halide,lglucin/Halide,mcanthony/Halide,smxlong/Halide,delcypher/Halide,tdenniston/Halide,mcanthony/Halide,aam/Halide,tdenniston/Halide,adasworks/Halide,gchauras/Halide,gchauras/Halide,rodrigob/Halide,damienfir/Halide,myrtleTree33/Halide,dan-tull/Halide,tdenniston/Halide,dan-tull/Halide,myrtleTree33/Halide,myrtleTree33/Halide,gchauras/Halide,kgnk/Halide,ronen/Halide,tdenniston/Halide,ronen/Halide,kenkuang1213/Halide,damienfir/Halide,kenkuang1213/Halide,rodrigob/Halide,dan-tull/Halide,aam/Halide,myrtleTree33/Halide,smxlong/Halide,dougkwan/Halide,jiawen/Halide,dan-tull/Halide,aam/Halide,fengzhyuan/Halide,aam/Halide,adasworks/Halide,ayanazmat/Halide,dougkwan/Halide,ayanazmat/Halide