Compiler should be smarter about immutable closured variables

[asterite]

I couldn't find a way to reduce the title's size, but basically:

def capture(&block)
  block
end

x = rand < 0.5 ? 1 : nil
if x
  capture do
    x + 1 # undefined method '+' for Nil (compile-time type is (Int32 | Nil))
  end
end

Even though we check that x is not nil, and it's never modified again, the compiler complains because right now any closured variable has all types all the time (they are not affected by type flow). This should be improved by noticing that x doesn't change. It's not trivial because if inside a while or another block it gets tricky, but should be doable.

[asterite]

This should also work:

x = 1
x = "hello"

capture do
  x # here x is known to always be a String, never an Int32
end