nir: Add a passes for nir_intrinsic_convert_alu_types

nir: Add a passes for nir_intrinsic_convert_alu_types

This adds primarily two passes:  One is a lowering pass which turns
these conversion intrinsics into a series of ALU ops.  The other is an
optimization pass which attempt to simplify the conversion whenever
possible in the hopes that we can turn it into a "normal" conversion op
which doesn't need special treatment.

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>