[llvm] [TTI][TLI] Support scalable immediates with isLegalAddImmediate (PR #84173)

[David Green via llvm-commits]

davemgreen wrote:

Can you remind us where these functions are used? It might be simpler to have separate methods and move the scalable vs normal immediate check out a level.

https://github.com/llvm/llvm-project/pull/84173