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

[Akshay Deodhar via llvm-commits]

akshayrdeodhar wrote:

Erroneously pushed a partial rebase, reviewers were added due to merge conflicts. Have removed them now, and have restored the PR to be based off an older version of main.

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