intel/fs: Retype dest to match value in read[First]Invocation

intel/fs: Retype dest to match value in read[First]Invocation

This is what we really wanted all along.  Always retyping to D works
because that's what get_nir_src() always gives us, at least for 32-bit
types.  The SPIR-V variants of these operations accept arbitrary types
and we need this if we're going to handle 64 or 16-bit values.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>