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

[Graham Hunter via llvm-commits]

huntergr-arm 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.

I'm modifying these to handle the uses in LoopStrengthReduce, specifically for GenerateReassociationsImpl and GenerateCrossUseConstantOffsets.

Since we're just asking about a single immediate (rather than a whole addressing mode, as in PR #83255 ) then it's fine to make a second interface. Will do so.

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