[LV] Add LoopVectorize/VPlan subdirectory for VPlan printing tests. (#180611) Add a new VPlan subdirectory as common place for tests checking VPlan printing. It contains a lit.local.cfg that only runs the tests when assertions are enabled. This removes the need to add explicit REQUIRES: asserts to VPlan tests. PR: https://github.com/llvm/llvm-project/pull/180611 GitOrigin-RevId: 8e1d5ec534a68a5a1c0667dfe9e4333143e07888