[InstCombine] allow vector splats for add+and with high-mask

There might be a better way to specify the pre-conditions,
but this is hopefully clearer than the way it was written:
https://rise4fun.com/Alive/Jhk3

  Pre: C2 < 0 && isShiftedMask(C2) && (C1 == C1 & C2)
  %a = and %x, C2
  %r = add %a, C1
  =>
  %a2 = add %x, C1
  %r = and %a2, C2