implement conditional constant/copy propagation

[DenialAdams]

https://www.cs.utexas.edu/~lin/cs380c/wegman.pdf

proc foo {
  x = false;
  if x {
     y = 10;
  else {
     y = 11;
  end
  use(y);
}

we should fold 11 into use(y)

[DenialAdams]

this did happen