nir: Add builder helpers for OpenCL type conversions

nir: Add builder helpers for OpenCL type conversions

Most of these were originally written by Daniel Stone in the Microsoft
ClOn12 branch, reworked by Jesse Natalie, fixed by Boris Brezillon, and
possibly touched by others along the way.  Unfortunately, none of that
is in the commit history thanks to living in the CLOn12 branch.

I ported them to mesa master and further reworked things for better
cosmetics.  In particular,

 1. They now live in a builder helper rather than in vtn_alu.c.

 2. Instead of looping inside each builder helper, we just trust NIR
    vector instructions to handle vectors.

 3. Lots of re-arranging of the helpers for clarity, better asserting,
    and better re-use with the upcoming lowering pass.

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>