Simplify: Fixes IndexError using cse with RootOf

[sachin-4099]

Fixes #18203

Initially:

In [1]: eq = CRootOf(x**5 + 11*x - 2, 0) + CRootOf(x**5 + 11*x - 2, 1)
In [2]: cse(eq)
---------------------------------------------------------------------------
IndexError Traceback (most recent call last)
<ipython-input-3-cf150e6c3960> in <module>
----> 1 cse(eq)

Now the issue is fixed.

Testcase has also been added.

• simplify
  • Fixed IndexError using cse with RootOf

[sympy-bot]

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• simplify
  • Fixed IndexError using cse with RootOf (#18273 by @sachin-4099)

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Fixes #18203 

**Initially:**

`In [1]: eq = CRootOf(x**5 + 11*x - 2, 0) + CRootOf(x**5 + 11*x - 2, 1)`
`In [2]: cse(eq)`
`---------------------------------------------------------------------------`
`IndexError                                Traceback (most recent call last)`
`<ipython-input-3-cf150e6c3960> in <module>`
`----> 1 cse(eq)`

Now the **issue is fixed**.

**Testcase has also been added.**

<!-- BEGIN RELEASE NOTES -->
* simplify
   *  Fixed IndexError using cse with RootOf

<!-- END RELEASE NOTES -->

Update

The release notes on the wiki have been updated.

[sachin-4099]

Please review the PR @jksuom @oscarbenjamin.
If any changes are required please suggest
Otherwise the PR is ready to be merged.

[codecov]

Codecov Report

Merging #18273 into master will increase coverage by 0.174%.
The diff coverage is 100%.

@@              Coverage Diff              @@
##            master    #18273       +/-   ##
=============================================
+ Coverage   74.817%   74.991%   +0.174%     
=============================================
  Files          638       649       +11     
  Lines       166312    167382     +1070     
  Branches     39111     39394      +283     
=============================================
+ Hits        124431    125523     +1092     
+ Misses       36358     36317       -41     
- Partials      5523      5542       +19

[jksuom]

Thanks, this looks good.