when the concerned ARM instruction is doing something perfectly capable by the software to just score 1-2% performance improvements, it is definitely CISC based on the definitions you listed above.
This Javascript related instruction is complex in terms of a bitwise operator on a single value, however complex it is still a simple operation; CISC-ness generally relates more to accessing memory or interacting with the hardware in complex ways.
From the definition given:
CISC designs on the other hand happily encode large, arbitrarily complex operations that take unbounded amounts of time, and have very strong memory models
The operation it accelerates is very simple, its just not really required much outside of javascript. It is already identical to an existing instruction, just with slightly different overflow behavior that javascript relies on.
