optimize access of temporary array of 2 elements

[rotateright]

I was curious if we could optimize code as suggested in https://reviews.llvm.org/D137688 .
I'm not sure which passes are responsible, but it's a mess on a reduced example:

int src(int i, int x, int y) {
  if (i != 0 && i != 1)
    __builtin_unreachable();

  int a[2] = {x, y};  
  return a[1 - i];
}

int foo(int i, int x, int y) {
  if (i != 0 && i != 1)
    __builtin_unreachable();

  int a[2] = {x, y};  
  return i ? a[0] : a[1];
}

Bad in IR:
https://alive2.llvm.org/ce/z/tqrgci

And that carries over to bad asm:
https://godbolt.org/z/exc56dW4z