Fixed generation of shift operators in OpenCL C for vector types.
Added test for vectorized div/mod in GPU targets.
Fixed comments in vectorized div/mod test.
Merge branch 'master' of https://github.com/halide/Halide into no-shift-ocl-c
Merge pull request #550 from halide/no-shift-ocl-c

Don't lower Div/Mod to shift/and in OpenCL C