Test case:
%vec = type <3 x float>
define %vec @vecdiv( %vec %p1, %vec %p2)
{
            %result = fdiv %vec %p1, %p2
            ret %vec %result
}
(With different vector lengths)

Behavior of current DAGTypeLegalizer::WidenVecRes_Binary on x86-64:

<3 x float>: widen to 4:
  concat(fdiv:v2f32, insert(undef:v2f32, fdiv:f32)) RIGHT, but yields incorrect code (bug elsewhere)
<5 x float>: widen to 8:
  concat(fdiv:v4f32, insert(v2f32, fdiv:f32), undef:v2f32, undef:v2f32) WRONG, but works (accident?).
<6 x float>: widen to 8:
  concat(fdiv:v4f32, fdiv:v2f32, undef:v2f32, undef:v2f32) WRONG, produces wrong result
<7 x float>: widen to 8:
  concat(fdiv:v4f32, fdiv:v2f32, insert(fdiv:f32):v2f32, undef:v2f32) WRONG, wrong result
<9 x float>: widen to 16:
  concat(fdiv:v4f32, fdiv:v4f32, insert(fdiv:f32):v2f32, undef:v4f32) WRONG, assert blows
<10 x float>: widen to 16:
  concat(fdiv:v4f32, fdiv:v4f32, concat(fdiv:v2f32, undef:v2f32):v4f32, undef:v4f32) RIGHT, right result
<11 x float>: widen to 16:
  concat(fdiv:v4f32, fdiv:v4f32, concat(fdiv:v2f32, insert(undef:v2f32, 1 x fdiv:f32)) RIGHT, right result
<12 x float>: widen to 16:
  concat(fdiv:v4f32, fdiv:v4f32, fdiv:v4f32, undef:v4f32), RIGHT, right result
<13 x float>: widen to 16:
  concat(fdiv:v4f32, fdiv:v4f32, fdiv:v4f32, insert(undef:v2f32, 1x fdiv:f32)) WRONG, assert blows
<14 x float>: widen to 16:
  concat(fdiv:v4f32, fdiv:v4f32, fdiv:v4f32, concat(fdiv:v2f32, undef:v2f32)) RIGHT, right result
<15 x float>: widen to 16:
  concat(fdiv:v4f32, fdiv:v4f32, fdiv:v4f32, concat(fdiv:v2f32, insert(undef:v2f32, 1x fdiv:f32))) RIGHT