[llvm] [AArch64] Optimize lowering of extending BUILD_VECTOR(EXTRACT_VECTOR_ELT()) (PR #212156)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

please can you split the DAGCombiner.cpp change into its own PR

https://github.com/llvm/llvm-project/pull/212156