[VPlan] Make canonical IV part of the region#156262

fhahn wants to merge 18 commits intollvm:mainllvm/llvm-project:mainfrom

fhahn:vplan-caniv-defined-by-region-dont-reset-start-valuefhahn/llvm-project:vplan-caniv-defined-by-region-dont-reset-start-valueCopy head branch name to clipboard