[llvm] [VPlan] Move auxiliary declarations out of VPlan.h (NFC). (PR #124104)

[David Sherwood via llvm-commits]

david-arm wrote:

Given the size of the changes in this patch I wonder if it's worth considering breaking down into a fewer smaller patches, if possible? I'm just thinking potential disruption to other developers. If all the changes in this patch depend upon each other that's fine though.

https://github.com/llvm/llvm-project/pull/124104