[llvm] [TTI] Support scalable offsets in getScalingFactorCost (PR #88113)

[Paul Walker via llvm-commits]

https://github.com/paulwalker-arm approved this pull request.

Based on the current definition of `TargetLoweringBase::AddrMode` I'm happy with this change.  As Sander points out, there's now multiple ways to represent the same thing, which is messy and so I'd love to see some follow on work to make `TargetLoweringBase::AddrMode` use the correct types, be that `StackOffset` if the current requirement of `BaseGV + BaseOffs + BaseReg + Scale*ScaleReg + ScalableOffset*vscale` is what we want, or something else if the actual goal is for `BaseOffs` to be fixed **OR** scalable.

https://github.com/llvm/llvm-project/pull/88113