mesa: new _mesa_remove_varying_reads() function

mesa: new _mesa_remove_varying_reads() function

We'll apply this function to GLSL vertex programs.  In GLSL it's legal to
read and write varying (output) vars in a vertex shader.  But reading from
an output register isn't supported by all hardware.  This routine examines
the vertex program for that condition and rewrites it to use temporary
registers where needed.