[AArch64][SelectionDAG] fix infinite loop caused by legalizing & combining CONCAT_VECTORS

Legalizing in `AArch64TargetLowering::LowerCONCAT_VECTORS()` and combining in `DAGCombiner::visitCONCAT_VECTORS()` could cause an infinite loop.
This commit fixes that issue by conditionally skipping the combining.

Fix https://github.com/llvm/llvm-project/issues/63322

Reviewed By: RKSimon, MaskRay

Differential Revision: https://reviews.llvm.org/D153316