[llvm] [VPlan] Construct initial once and pass clones to tryToBuildVPlan (NFC). (PR #141363)

[Nikita Popov via llvm-commits]

nikic wrote:

FYI this has some overhead, though not a lot: https://llvm-compile-time-tracker.com/compare.php?from=435d8b12efc1447b3c9f95ace2c57dae22ea486d&to=567b3172da2d52f5df70a37f3de06b7000b25968&stat=instructions:u I assume that's to be expected given what the change does.

https://github.com/llvm/llvm-project/pull/141363