nir/serialize: fix vec8 and vec16

nir/serialize: fix vec8 and vec16

Nir serializes uses nir_ssa_alu_instr_src_components in a few places to
determine how many components a src has, but that's not what this function
returns. It simply returns how many channels are used, which is still fine
for most of the code.

This was breaking code like this:

vec16 32 ssa_1 = intrinsic load_global
vec1  32 ssa_2 = fmax ssa_1.a, ssa_2.b

v2: make the 16bit encoding work for identify swizzles again

Signed-off-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>