arb_gpu_shader_fp64: fix indirect array access test

Because we are writing to and reading from the array with the same uniform index Mesas NIR optimisations were smart enough to eliminate the array altogether. With an upcoming Mesa improvement to eliminate dead uniforms this was also causing the tests to fail because the uniform pick gets eliminated and the test can no longer set it.

With this change we add pick2 which will be set to the same value as pick but only used when reading from the array. With this NIR can no longer eliminate the array.