intel/fs: Use F32TO16/F16TO32 helpers in fquantize16 handling

intel/fs: Use F32TO16/F16TO32 helpers in fquantize16 handling

I originally thought that we were intentionally emitting the legacy
opcodes here to make them opaque to the optimizer, so that it wouldn't
eliminate the explicit type conversions, as they're actually required
to do the quantization.  But...we don't actually optimize those away
currently anyway.  So...go ahead and use the helpers for consistency.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Sagar Ghuge <sagar.ghuge@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21783>