Fix for Condition failed: op->args[0].as<StringImm>() running apps/glsl ...

...test #384 

> This fixes the assert in the test:
> Internal error at src/CodeGen_GPU_Host.cpp:182
> Condition failed: op->args[0].as<StringImm>()
>
> The op for the Call visitor is a glsl_texture_load and CodeGen_GPU_Host
> assumes that the first argument is a StringImm. The argument was originally
> assigned as one by the InjectOpenGLIntrinsics pass, but it appears that a
> CSE pass is changing it to an ExprNode.

The fix prevents the CSE pass from replacing StringImms.

The pull also includes updates to the build files for the apps/glsl test.