[llvm] [InstCombine] Skip redundant demanded element queries in insert chains (PR #205948)

[Antonio Frighetto via llvm-commits]

https://github.com/antoniofrighetto commented:

Could you please add compile-time measurements showing the benefit of the change (and which workload motivates this)? Should this check live inside the InsertElement case in SimplifyDemandedVectorElts() itself?

https://github.com/llvm/llvm-project/pull/205948