[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
38 files changed