fix[codegen]: same symbol jumpdest merge

[harkal]

What I did

Fixed assembly optimizer case where trying to optimize two consecutive JUMPDESTs with the same symbol results in no code change, while returning of True in changes.

How I did it

Made the optimization step return False as it should.

How to verify it

Through the included tests

Commit message

Updates the assembly optimizer to return `False` when trying to 
optimize two consecutive `JUMPDEST`s with the same symbol. 
It used to return `True` when no change was made.

+ unit tests

Description for the changelog

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[charles-cooper]

thank you!