Hi,
For the following test-case:

typedef float __attribute__((__vector_size__ (16))) F;
F foo (F a, F b)
{
  F v = (F) { 9 };
  return __builtin_shufflevector (v, v, 1, 0, 1, 2);
}

Compiling with -O2 results in following ICE:
foo.c: In function ‘foo’:
foo.c:6:10: internal compiler error: in decompose, at rtl.h:2314
    6 |   return __builtin_shufflevector (v, v, 1, 0, 1, 2);
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
0x7f3185 wi::int_traits<std::pair<rtx_def*, machine_mode>
>::decompose(long*, unsigned int, std::pair<rtx_def*, machine_mode>
const&)
        ../../gcc/gcc/rtl.h:2314
0x7f3185 wide_int_ref_storage<false,
false>::wide_int_ref_storage<std::pair<rtx_def*, machine_mode>
>(std::pair<rtx_def*, machine_mode> const&)
        ../../gcc/gcc/wide-int.h:1089
0x7f3185 generic_wide_int<wide_int_ref_storage<false, false>
>::generic_wide_int<std::pair<rtx_def*, machine_mode>
>(std::pair<rtx_def*, machine_mode> const&)
        ../../gcc/gcc/wide-int.h:847
0x7f3185 poly_int<1u, generic_wide_int<wide_int_ref_storage<false,
false> > >::poly_int<std::pair<rtx_def*, machine_mode>
>(poly_int_full, std::pair<rtx_def*, machine_mode> const&)
        ../../gcc/gcc/poly-int.h:467
0x7f3185 poly_int<1u, generic_wide_int<wide_int_ref_storage<false,
false> > >::poly_int<std::pair<rtx_def*, machine_mode>
>(std::pair<rtx_def*, machine_mode> const&)
        ../../gcc/gcc/poly-int.h:453
0x7f3185 wi::to_poly_wide(rtx_def const*, machine_mode)
        ../../gcc/gcc/rtl.h:2383
0x7f3185 rtx_vector_builder::step(rtx_def*, rtx_def*) const
        ../../gcc/gcc/rtx-vector-builder.h:122
0xfd4e1b vector_builder<rtx_def*, machine_mode,
rtx_vector_builder>::elt(unsigned int) const
        ../../gcc/gcc/vector-builder.h:253
0xfd4d11 rtx_vector_builder::build()
        ../../gcc/gcc/rtx-vector-builder.cc:73
0xc21d9c const_vector_from_tree
        ../../gcc/gcc/expr.cc:13487
0xc21d9c expand_expr_real_1(tree_node*, rtx_def*, machine_mode,
expand_modifier, rtx_def**, bool)
        ../../gcc/gcc/expr.cc:11059
0xaee682 expand_expr(tree_node*, rtx_def*, machine_mode, expand_modifier)
        ../../gcc/gcc/expr.h:310
0xaee682 expand_return
        ../../gcc/gcc/cfgexpand.cc:3809
0xaee682 expand_gimple_stmt_1
        ../../gcc/gcc/cfgexpand.cc:3918
0xaee682 expand_gimple_stmt
        ../../gcc/gcc/cfgexpand.cc:4044
0xaf28f0 expand_gimple_basic_block
        ../../gcc/gcc/cfgexpand.cc:6100
0xaf4996 execute
        ../../gcc/gcc/cfgexpand.cc:6835

IIUC, the issue is that fold_vec_perm returns a vector having float element
type with res_nelts_per_pattern == 3, and later ICE's when it tries
to derive element v[3], not present in the encoding, while trying to
build rtx vector
in rtx_vector_builder::build():
 for (unsigned int i = 0; i < nelts; ++i)
    RTVEC_ELT (v, i) = elt (i);

The attached patch tries to fix this by returning false from
valid_mask_for_fold_vec_perm_cst if sel has a stepped sequence and
input vector has non-integral element type, so for VLA vectors, it
will only build result with dup sequence (nelts_per_pattern < 3) for
non-integral element type.

For VLS vectors, this will still work for stepped sequence since it
will then use the "VLS exception" in fold_vec_perm_cst, and set:
res_npattern = res_nelts and
res_nelts_per_pattern = 1

and fold the above case to:
F foo (F a, F b)
{
  <bb 2> [local count: 1073741824]:
  return { 0.0, 9.0e+0, 0.0, 0.0 };
}

But I am not sure if this is entirely correct, since:
tree res = out_elts.build ();
will canonicalize the encoding and may result in a stepped sequence
(vector_builder::finalize() may reduce npatterns at the cost of increasing
nelts_per_pattern)  ?

PS: This issue is now latent after PR111648 fix, since
valid_mask_for_fold_vec_perm_cst with  sel = {1, 0, 1, ...} returns
false because the corresponding pattern in arg0 is not a natural
stepped sequence, and folds correctly using VLS exception. However, I
guess the underlying issue of dealing with non-integral element types
in fold_vec_perm_cst still remains ?

The patch passes bootstrap+test with and without SVE on aarch64-linux-gnu,
and on x86_64-linux-gnu.

Thanks,
Prathamesh