Identified with readability-delete-null-pointer.

Classof for most recipes directly supports VPValue, so there is no need
to call getDefiningRecipe when using isa/cast/dyn_cast.

Use getDefiningRecipe to future-proof the code. Split off from
https://github.com/llvm/llvm-project/pull/156262 as suggested.

EnableVPlanNativePath is declared in LoopVectorizationPlanner.h.

Identified with readability-redundant-declaration.

vputils::isSingleScalar is sufficient.

This reverts commit 8d29d09309654541fb2861524276ada6a3ebf84c.

There have been reports of mis-compiles
in https://github.com/llvm/llvm-project/pull/149706.

Revert while I investigate.

Split off to clarify naming, as suggested in
https://github.com/llvm/llvm-project/pull/156262.

Move narrowInterleaveGroups to to general VPlan optimization stage.

To do so, narrowInterleaveGroups now has to find a suitable VF where all
interleave groups are consecutive and saturate the full vector width.

If such a VF is found, the original VPlan is split into 2:
 a) a new clone which contains all VFs of Plan, except VFToOptimize, and
 b) the original Plan with VFToOptimize as single VF.

The original Plan is then optimized. If a new copy for the other VFs has
been created, it is returned and the caller has to add it to the list of
candidate plans.

Together with https://github.com/llvm/llvm-project/pull/149702, this
allows to take the narrowed interleave groups into account when
computing costs to choose the best VF and interleave count.

One example where we currently miss interleaving/unrolling when
narrowing interleave groups is https://godbolt.org/z/Yz77zbacz

PR: https://github.com/llvm/llvm-project/pull/149706

Follow up on 7c4f188 ([LV] Support multiplies by constants when forming
scaled reductions), introducing m_APInt, and improving code around
canConstantBeExtended: we change canConstantBeExtended to take an APInt.