nir/lower_io_to_temporaries: also fixup load_deref in FS
Without this fixup, load_deref might be replaced by loading from the temporary that was loaded by using one of the interpolate_at* instructions.
Closes #4808
Remark: I'm not, but it might be that this makes the whole lowering pass useless for FS inputs. A better approach might be to track which interpolators are used on a variable (similar to what I propose in !10887)