Control flow analysis

[viega]

Do control flow analysis to be able to detect statically when a variable can be referenced before assignment. Right now, this is a runtime error, and it doesn't need to be.

[viega]

Also should make sure w/ this analysis it's fine to have:

if (x) {
  y = 2
}
else {
  y = 4
}
echo(string(y))

Right now, you'd need to put y = 0 at the top.