ac: fix nir_op_f2f64

ac: fix nir_op_f2f64

Without this we get the error "FPExt only operates on FP" when
converting the following:

   vec1 32 ssa_5 = b2f ssa_4
   vec1 64 ssa_6 = f2f64 ssa_5

Which results in:

   %44 = and i32 %43, 1065353216
   %45 = fpext i32 %44 to double

With this patch we now get:

   %44 = and i32 %43, 1065353216
   %45 = bitcast i32 %44 to float
   %46 = fpext float %45 to double

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>