Add explicit conversion of mod by powers of two to bit masking.
Add explicit conversion of mod by powers of two to bit masking.

Not necessary for llvm 3.2 and 3.3, but llvm trunk's treatment of vector
urem seems flaky of late.


Former-commit-id: 3d8a88d80a0d3dc9a78433d0692c261c9add25d2
Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide