if + var does not properly restrict type when used in closure

[straight-shoota]

def foo(&block : -> )
  block.call
end

base = nil
foo do
  if base
    base.upcase # Error: undefined method 'upcase' for Nil
  end
end

The same code works if base is assigned to a fresh variable inside the block or the block is inlined (yield instead of block.call).

[asterite]

Duplicate #3093