nir: Add a conversion and rounding intrinsic

This new intrinsic is capable of handling the full range of conversions
from OpenCL including rounding modes and possible saturation.  The
intention is that we'll emit this intrinsic directly from spirv_to_nir
and then lower it to ALU ops later.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6945>
5 files changed