[llvm] [AMDGPU][NFC] Refactor SCC optimization (PR #165871)

[Juan Manuel Martinez Caamaño via llvm-commits]

jmmartinez wrote:

> @jmmartinez I don't think optimizeSCC will be used outside of optimizeCompareInstr, so I would prefer to keep it a lambda here.

And to a static function only visible inside `SIIntrInfo.cpp` ? I think `RI` is the only member from `this` that's being accessed.

Having lambdas taking other lambdas by copy seems odd (specially since something simpler would do the job just as well).

https://github.com/llvm/llvm-project/pull/165871