[VPlan] Create header phis once, after constructing VPlan0 (NFC). (#168291)
Together with https://github.com/llvm/llvm-project/pull/168289 &
https://github.com/llvm/llvm-project/pull/166099 we can construct header
phis once up front, after creating VPlan0, as the
induction/reduction/first-order-recurrence classification applies across
all VFs.
Depends on https://github.com/llvm/llvm-project/pull/168289 &
https://github.com/llvm/llvm-project/pull/166099
PR: https://github.com/llvm/llvm-project/pull/168291