glsl: Add ir_unop_f2fmp

glsl: Add ir_unop_f2fmp

This is the same as ir_unop_f2f16 except that it comes with a promise
that it is safe to optimise it out if the result is immediately
converted back to float32 again. Normally this would be a lossy
operation but it is safe to do if the conversion was generated as part
of the precision lowering pass.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3929>