Extend Style/SelfAssignment beyond variables

[vlad-pisanov]

Is there a reason why Style/SelfAssignment only works with variables, as opposed to general expressions? I.e. could we lint for something like:

# bad
hash['foo'] = hash['foo'] + 42
user.name = user.name + " Jr."

# good
hash['foo'] += 42
user.name += " Jr."

[ydakuka]

And (see ops):

# bad

x = x % 2
x = x ^ 2
x = x << 2
x = x >> 2

# good

x %= 2
x ^= 2
x <<= 2
x >>= 2

[koic]

The main issue was resolved by #12415, and the derivative will be resolved by #12421.