[llvm] [RISCV] Introduce the RISCVLoopIdiomRecognizePass (PR #92441)

[Petr Penzin via llvm-commits]

ppenzin wrote:

> There are differences in how we expand the code. Maybe we can use the TTI hooks the middle loop vectorizer uses to have different strategies for AArch64 and RISC-V. If that gets too complicated we could have a common base class that AArch64 and RISC-V both inherit their own passes from so they can share the code that makes sense.

Yeah, that makes sense.

https://github.com/llvm/llvm-project/pull/92441