nir: Add optimization for doing removing f16/f32 conversions

nir: Add optimization for doing removing f16/f32 conversions

This eliminates conversions between f16 and f32 where possible. We can
always remove an upcast followed by a down cast, that is:

  f2f16 ( f2f32 (a) )  ->  a
  f2fmp ( f2f32 (a) )  ->  a

In the other direction, f2f16 loses precision and can't be undone by a
f2f32.  However, by definition it's always safe to elminate f2fmp:

  f2f32 ( f2fmp (a) )  ->  a

v2. [Neil Roberts (nroberts@igalia.com)]

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3822>