Assignment in every branch still reconstructs original assignment

[Ellpeck]

x <- 1
if(x) {
  if(y) {
    x <- 3
  } else {
    x <- 2
  }
} else {
  x <- 4
}
print(x)

results in the first line (x <- 1) still being part of the slice, even though x is overwritten in every branch.

[EagleoutIce]

On a second look, this is the fault of the piece of code.
the first condition if(x) causes x <- 1 to be included.
Please re-insert the test as a part of #800, but modify the condition accordingly @Ellpeck.