[llvm] [AArch64][CostModel] Consider the cost of const vector (PR #117539)

[Sushant Gokhale via llvm-commits]

sushgokh wrote:

> Thanks for the patch, I planned adding constant vector materialization cost myself for some time. Unfortunately, SLP part of this patch is wrong. The only place, where the constant materialization cost must be used in SLPVectorizer are `InstructionCost getBuildVectorCost(ArrayRef<Value *> VL, Value *Root)` function and `InstructionCost BoUpSLP::getGatherCost` function, all other cost estimations must be in TTI

Thanks @alexey-bataev . Will update the patch.

https://github.com/llvm/llvm-project/pull/117539