Hi Richard,
For the following reduced test-case taken from PR:

#include "arm_sve.h"
svuint32_t l() {
  alignas(16) const unsigned int lanes[4] = {0, 0, 0, 0};
  return svld1rq_u32(svptrue_b8(), lanes);
}

compiling with -O3 -mcpu=generic+sve results in following ICE:
during GIMPLE pass: fre
pr110280.c: In function 'l':
pr110280.c:5:1: internal compiler error: in eliminate_stmt, at
tree-ssa-sccvn.cc:6890
    5 | }
      | ^
0x865fb1 eliminate_dom_walker::eliminate_stmt(basic_block_def*,
gimple_stmt_iterator*)
        ../../gcc/gcc/tree-ssa-sccvn.cc:6890
0x120bf4d eliminate_dom_walker::before_dom_children(basic_block_def*)
        ../../gcc/gcc/tree-ssa-sccvn.cc:7324
0x120bf4d eliminate_dom_walker::before_dom_children(basic_block_def*)
        ../../gcc/gcc/tree-ssa-sccvn.cc:7257
0x1aeec77 dom_walker::walk(basic_block_def*)
        ../../gcc/gcc/domwalk.cc:311
0x11fd924 eliminate_with_rpo_vn(bitmap_head*)
        ../../gcc/gcc/tree-ssa-sccvn.cc:7504
0x1214664 do_rpo_vn_1
        ../../gcc/gcc/tree-ssa-sccvn.cc:8616
0x1215ba5 execute
        ../../gcc/gcc/tree-ssa-sccvn.cc:8702

cc1 simplifies:
  lanes[0] = 0;
  lanes[1] = 0;
  lanes[2] = 0;
  lanes[3] = 0;
  _1 = { -1, ... };
  _7 = svld1rq_u32 (_1, &lanes);

to:
  _9 = MEM <vector(4) unsigned int> [(unsigned int * {ref-all})&lanes];
  _7 = VEC_PERM_EXPR <_9, _9, { 0, 1, 2, 3, ... }>;

and then fre1 dump shows:
Applying pattern match.pd:8675, generic-match-5.cc:9025
Match-and-simplified VEC_PERM_EXPR <_9, _9, { 0, 1, 2, 3, ... }> to {
0, 0, 0, 0 }
RHS VEC_PERM_EXPR <_9, _9, { 0, 1, 2, 3, ... }> simplified to { 0, 0, 0, 0 }

The issue seems to be with the following pattern:
(simplify
 (vec_perm vec_same_elem_p@0 @0 @1)
 @0)

which simplifies above VEC_PERM_EXPR to:
_7 = {0, 0, 0, 0}
which is incorrect since _9 and mask have different vector lengths.

The attached patch amends the pattern to simplify above VEC_PERM_EXPR
only if operand and mask have same number of elements, which seems to fix
the issue, and we're left with the following in .optimized dump:
  <bb 2> [local count: 1073741824]:
  _2 = VEC_PERM_EXPR <{ 0, 0, 0, 0 }, { 0, 0, 0, 0 }, { 0, 1, 2, 3, ... }>;
  return _2;

code-gen:
l:
        mov     z0.b, #0
        ret

Patch is bootstrapped+tested on aarch64-linux-gnu.
OK to commit ?

Thanks,
Prathamesh