nir: correct use of identity check in python

nir: correct use of identity check in python

Python has the identity operator `is`, and the equality operator `==`.
Using `is` with strings sometimes works in CPython due to optimizations
(they have some kind of cache), but it may not always work.

Fixes: 96c4b135e34d0804e41bfbc28fc1b5050c49d71e
       ("nir/algebraic: Don't put quotes around floating point literals")
Reviewed-by: Matt Turner <mattst88@gmail.com>