[llvm] [AArch64] Support scalable offsets with isLegalAddressingMode (PR #83255)

[David Green via llvm-commits]

davemgreen wrote:

This sounds like a nicer approach that adding the new type. Did you consider just having another `int64_t ScalableOffset` separate from the `Offset`? It might not really come up very often that an architecture has both at the same time, but it would be a little more general and fit into AddressingModes being a collection of things all added together. 

https://github.com/llvm/llvm-project/pull/83255