[llvm] [SCEV] Simplify SCEVExpr for PHI to SCEV for operand if operands are identical (PR #115945)

[Akshay Deodhar via llvm-commits]

akshayrdeodhar wrote:

> Can we widen %srcLength and %srcIndex.0.be as well to fully eliminate all uses of %srcIndex.03?

This will be an improvement to IndVarSimplify- I'm not familiar with that pass - will it be fine if I land this MR independently of the improvement?

https://github.com/llvm/llvm-project/pull/115945