nv50/ir/nir: implement load_(interpolated_)input/output

nv50/ir/nir: implement load_(interpolated_)input/output

v3: and load_output
v4: use smarter getIndirect helper
    use new getSlotAddress helper
v5: don't use const_offset directly
    fix for indirects
v6: add support for interpolateAt
v7: fix compiler warnings
    add load_barycentric_sample
    handle load_output for fragment shaders
v8: set info->prop.fp.readsSampleLocations for at_sample interpolation
    don't require C++11 features
v9: convert to C++ style comments

Signed-off-by: Karol Herbst <kherbst@redhat.com>