[Bug target/98532] Use load/store pairs for 2-element vector in memory permutes

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98532

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
  _1 = BIT_FIELD_REF <*a_4(D), 64, 64>;
  _2 = BIT_FIELD_REF <*a_4(D), 64, 0>;
  tmp_5 = {_1, _2};

note this is another case where we'd like to improve forwprop to canonicalize
this to

  _1 = *a_4(D);
  tmp_5 = VEC_PERM <_1, _1, { 1, 0 }>;

which is more "SSA friendly".  That would lead to what clang generates
I guess (and be maybe easier to combine from to ldp/stp).