[VPlan] Hook IR blocks into VPlan during skeleton creation (NFC) (#114292)

As a first step to move towards modeling the full skeleton in VPlan,
start by wrapping IR blocks created during legacy ske

[VPlan] Hook IR blocks into VPlan during skeleton creation (NFC) (#114292)

As a first step to move towards modeling the full skeleton in VPlan,
start by wrapping IR blocks created during legacy skeleton creation in
VPIRBasicBlocks and hook them into the VPlan. This means the skeleton
CFG is represented in VPlan, just before execute. This allows moving
parts of skeleton creation into recipes in the VPBBs gradually.

Note that this allows retiring some manual DT updates, as this will be
handled automatically during VPlan execution.

PR: https://github.com/llvm/llvm-project/pull/114292

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[VPlan] Implement unrolling as VPlan-to-VPlan transform. (#95842)

This patch implements explicit unrolling by UF as VPlan transform. In
follow up patches this will allow simplifying VPTransform st

[VPlan] Implement unrolling as VPlan-to-VPlan transform. (#95842)

This patch implements explicit unrolling by UF as VPlan transform. In
follow up patches this will allow simplifying VPTransform state (no need
to store unrolled parts) as well as recipe execution (no need to
generate code for multiple parts in an each recipe). It also allows for
more general optimziations (e.g. avoid generating code for recipes that
are uniform-across parts).

It also unifies the logic dealing with unrolled parts in a single place,
rather than spreading it out across multiple places (e.g. VPlan post
processing for header-phi recipes previously.)

In the initial implementation, a number of recipes still take the
unrolled part as additional, optional argument, if their execution
depends on the unrolled part.

The computation for start/step values for scalable inductions changed
slightly. Previously the step would be computed as scalar and then
splatted, now vscale gets splatted and multiplied by the step in a
vector mul.

This has been split off https://github.com/llvm/llvm-project/pull/94339
which also includes changes to simplify VPTransfomState and recipes'
::execute.

The current version mostly leaves existing ::execute untouched and
instead sets VPTransfomState::UF to 1.

A follow-up patch will clean up all references to VPTransformState::UF.

Another follow-up patch will simplify VPTransformState to only store a
single vector value per VPValue.

PR: https://github.com/llvm/llvm-project/pull/95842

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[VPlan] Add getSCEVExprForVPValue util, use to get trip count SCEV (NFC) (#94464)

Add a new getSCEVExprForVPValue utility which can be used to get a SCEV
expression for a VPValue. The initial imple

[VPlan] Add getSCEVExprForVPValue util, use to get trip count SCEV (NFC) (#94464)

Add a new getSCEVExprForVPValue utility which can be used to get a SCEV
expression for a VPValue. The initial implementation only returns SCEVs
for live-in IR values (by constructing a SCEV based on the live-in IR
value) and VPExpandSCEVRecipe. This is enough to serve its first use,
getting a SCEV for a VPlan's trip count, but will be extended in the
future.

It also removes createTripCountSCEV, as the new helper can be used to
retrieve the SCEV from the VPlan.

PR: https://github.com/llvm/llvm-project/pull/94464

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VPlan: factor out VPlanUtils into its own file (NFC) (#105857)