spirv: Mark phis as mediump instead of directly lowering them to 16 bit.

spirv: Mark phis as mediump instead of directly lowering them to 16 bit.

This reverts commit 6f25d45877a1e1a7ac6250a7d051d33485e0cba7, replacing it
with GLSL_PRECISION_MEDIUM.  The previous commit ended up not being the
right approach, as it affected only nir vars for spirv phis and not other
nir vars, and we want a tool that does both.  The new
nir_lower_mediump_vars pass can do that for you.

No fossil-db change for my angle fossils run on radv.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18259>