The OpenGL and OpenGL ES specs require that implementations clamp the
value of gl_PointSize to an implementation-depedent range. This pass
is useful for any GPU hardware that doesn't do this automatically
for either one or both sides of the range, such as V3D.

v2:
 - Turn into a generic NIR pass (Eric).
 - Make the pass work before lower I/O so we can use the deref variable
   to inspect if we are writing to gl_PointSize (Eric).
 - Make the pass take the range to clamp as parameter and allow it
   to clamp to both sides of the range or just one side.
 - Make the pass report progress.

v3:
 - Fix copyright header (Eric)
 - use fmin/fmax instead of bcsel to clamp (Eric)

Reviewed-by: Eric Anholt <eric@anholt.net>