Workaround for an llvm performance bug
Workaround for an llvm performance bug

loop-strength-reduce was choking on long chains of select ir nodes. This
uses bitmasking for integer min and max instead of select. They get converted
to select anyway, but apparently the form they appear in no longer tickles
the bug?


Former-commit-id: af0a3c30309242c0c6b1d2062b2d6bda2c6f6012
darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide