spirv: handle phis decorated with RelaxedPrecision

spirv: handle phis decorated with RelaxedPrecision

If the driver can do 16-bit ALU ops, then store RelaxedPrecision phi
values into 16-bit NIR variables with downconverts/upconverts on the way
in/out.

This has no impact on shader-db on freedreno (not that we have a ton of
GLES content there), but it does cause an ANGLE-translated CTS shader on
vulkan to get consistent conversions between two copies of a value, and
avoid a test bug.

Reviewed-by: Emma Anholt <emma@anholt.net>
Closes: #6585
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14018>