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

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

I'm confused as to how this will work - how do you indicate when to test for a legal scalar or scalable immediate? Suppose you want to know if 0 is legal?

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