nir: Clean up nir_op_is_vec() and its callers

The nir_op_is_vec() helper I added in 842338e2f0 ("nir: Add a
nir_op_is_vec helper") treats nir_op_mov as a vec even though the
semanitcs of the two are different.  In retrospect, this was a mistake
as the previous three fixup commits show.  This commit splits the helper
into two: nir_op_is_vec() and nir_op_is_vec_or_mov() and uses the
appropriate helper at each call site.  Hopefully, this rename will
encurage any future users of these helpers to think about nir_op_mov as
separate from nir_op_vecN and we can avoid these bugs.

Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24704>

src/gallium/auxiliary/nir/nir_to_tgsi.c

@@ -570,9 +570,6 @@ ntt_extract_const_src_offset(nir_src *src)
       /* We'd like to reuse nir_scalar_chase_movs(), but it assumes SSA and that
        * seems reasonable for something used in inner loops of the compiler.
        */
-      if (!nir_alu_instr_is_copy(alu))
-         return 0;
-
       if (alu->op == nir_op_mov) {
          s.def = alu->src[0].src.ssa;
          s.comp = alu->src[0].swizzle[s.comp];