Simplified a check in _getitem_RepMatrix

[wermos]

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Brief description of what is fixed or changed

While investigating #26588, I noticed that there was a check that was awkwardly written. I simplified the check to make it easier to read and understand.

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#### References to other Issues or PRs
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#### Brief description of what is fixed or changed

While investigating #26588, I noticed that there was a check that was awkwardly written. I simplified the check to make it easier to read and understand.

#### Other comments


#### Release Notes

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[oscarbenjamin]

This change does not work for symbolic indices:

In [54]: i, j = symbols('i, j')

In [55]: M = Matrix([[1, 2], [3, 4]])

In [56]: M[i,j]
Out[56]: 
⎛⎡1  2⎤⎞      
⎜⎢    ⎥⎟[i, j]
⎝⎣3  4⎦⎠      

In [57]: M[i,j].subs({i:1,j:0})
Out[57]: 3