Avoid overlapping regions when extract similar=True

[brendanator]

If similar regions are found during extract method that overlap, the refactored code is invalid

For example, if the first two lines of func are extracted

def func(p):
    a = p
    b = a
    c = b
    d = c
    return d

the following is produced

def func(p):
    b = new_func(p)c = new_func(a)d = new_func(b)
    b = new_func(p)
    d = new_func(b)
    return d
def new_func(p):
    a = p
    b = a
    return b

With this PR a correct refactoring is performed

def func(p):
    b = new_func(p)
    d = new_func(b)
    return d
def new_func(p):
    a = p
    b = a
    return b

[soupytwist]

Looks good, thank you for debugging + fixing this!