Bad SLPVectorization shufflevector replacement, resulting in write to wrong memory location

We see that it might otherwise do:

  %10 = getelementptr {}**, <2 x {}***> %9, <2 x i32> <i32 10, i32 4>
  %11 = bitcast <2 x {}***> %10 to <2 x i64*>
...
  %27 = extractelement <2 x i64*> %11, i32 0
  %28 = bitcast i64* %27 to <2 x i64>*
  store <2 x i64> %22, <2 x i64>* %28, align 4, !tbaa !2

Which is an out-of-bounds store (the extractelement got offset 10
instead of offset 4 as intended). With the fix, we correctly generate
extractelement for i32 1 and generate correct code.

Differential Revision: https://reviews.llvm.org/D106613