nir: Fix nir_lower_alu_to_scalar's instr filtering.

nir: Fix nir_lower_alu_to_scalar's instr filtering.

It was checking if the dest or src[0] SSA values were vectors, rather than
whether the ALU op was using the source as a vector resulting in a
nir_fdot4 making it through to vc4 and v3d:

vec1 32 ssa_6 = fdot4 ssa_4.xxxx, ssa_5

Fixes: c1cffa4249ca ("nir/alu_to_scalar: Use the new NIR lowering framework")
v2: Use Jason's recommendation to look at input_sizes.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>