fix: return an error early for unsupported string binary operations

[edragain2nd]

fixes #23338

the reproduce code can actually reach the never!() macro in function eval_rvalue()

if *op != BinOp::Eq {
    never!("Only eq is builtin for `str`");
}

so evaluation continues and cause an out-of-bounds slice access

and the if statement means no only the shift operation(mentioned in the issue), any non-eq operation can cause the same panic. (like + in the test case)

I think the panic case is rare, so adding an early return would be valuable instead of modifying slice access which is used specifically for Eq operation.

[ChayimFriedman2]

Yet again MIR we should have rejected for errors in inference... But that what we got for now.

Thanks!

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