nir: move pixel_center_integer/origin_upper_left to shader_info.fs

New version of the thread that I sent recently, showing two initial versions to solve the regression I found on MR #144 (closed):

https://lists.freedesktop.org/archives/mesa-dev/2019-February/214808.html

This MR includes a v2 of the second option. It includes Jason's feedback plus some extra cleaning-ups that I found while re-checking the GLSL linker (like removing PixelCenterInteger/OriginUpperLeft from gl_program, as that info is tracked at gl_program.info.fs.xxx). Due that, it touches a lot of places on Mesa.

This MR also includes a second patch, that removes pixel_center_integer/origin_upper_left from the ir variable. Basically because that is already tracked in a lot of places, so it is not really needed. It is on a different patch because I initially though of it as something optional, as it is not really required to fix the regression. But after finishing it, the only reason to keep it as a different patch is to make easier the review. I think that it would be better to squash both patches, although I don't have a strong opinion, I would let the reviewer give his opinion.