(NFC modulo debug output changes)

Add generic helper to print phi operands (incoming values) together with
their incoming blocks.

As more and more transforms are added, keeping the incoming blocks of
phis becomes more important. Print incoming blocks via VPPhiAcessors, to
make debugging easier.

Update initial VPlan construction to include exit conditions and edges.

The loop region is now first constructed without entry/exiting. Those
are set after inserting the region in the CFG, to preserve the original
predecessor/successor order of blocks.

For now, all early exits are disconnected before forming the regions,
but a follow-up will update uncountable exit handling to also happen
here. This is required to enable VPlan predication and remove the
dependence any IR BBs
(https://github.com/llvm/llvm-project/pull/128420).

PR: https://github.com/llvm/llvm-project/pull/137709

Further simplify VPlan CFG builder by moving introduction of inner
regions to a VPlan transform, building on
https://github.com/llvm/llvm-project/pull/128419.

The HCFG builder now only constructs plain CFGs. I will move it to
VPlanConstruction as follow-up.

Depends on https://github.com/llvm/llvm-project/pull/128419.

PR: https://github.com/llvm/llvm-project/pull/129402

Create an empty VPlan first, then let the HCFG builder create a plain
CFG for the top-level loop (w/o a top-level region). The top-level
region is introduced by a separate VPlan-transform. This is instead of
creating the vector loop region before building the VPlan CFG for the
input loop.

This simplifies the HCFG builder (which should probably be renamed) and
moves along the roadmap ('buildLoop') outlined in [1].

As follow-up, I plan to also preserve the exit branches in the initial
VPlan out of the CFG builder, including connections to the exit blocks.

The conversion from plain CFG with potentially multiple exits to a
single entry/exit region will be done as VPlan transform in a follow-up.

This is needed to enable VPlan-based predication. Currently early exit
support relies on building the block-in masks on the original CFG,
because exiting branches and conditions aren't preserved in the VPlan.
So in order to switch to VPlan-based predication, we will have to
preserve them in the initial plain CFG, so the exit conditions are
available explicitly when we convert to single entry/exit regions.

Another follow-up is updating the outer loop handling to also introduce
VPRegionBlocks for nested loops as transform. Currently the existing
logic in the builder will take care of creating VPRegionBlocks for
nested loops, but not the top-level loop.

[1]
https://llvm.org/devmtg/2023-10/slides/techtalks/Hahn-VPlan-StatusUpdateAndRoadmap.pdf

PR: https://github.com/llvm/llvm-project/pull/128419

Update HCFG builder to preserve the original latch block of the initial
VPlan, ensuring there is always a latch.

It also skips creating the BranchOnCond for the latch of the top-level
loop, instead of removing it later. Exiting via the latch is controlled
by later recipes.

This further unifies HCFG construction and prepares for use to also
build an initial VPlan (VPlan0) for inner loops.

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

This split off changes for more complex CFGs in VPlan from both
    https://github.com/llvm/llvm-project/pull/114292
    https://github.com/llvm/llvm-project/pull/112138

This simplifies their respective diffs.

Update VPlan to include the scalar loop header. This allows retiring
VPLiveOut, as the remaining live-outs can now be handled by adding
operands to the wrapped phis in the scalar loop header.

Note that the current version only includes the scalar loop header, no
other loop blocks and also does not wrap it in a region block.

PR: https://github.com/llvm/llvm-project/pull/109975

Update buildPlainCFG to re-use the vector and latch VPBBs created as
part of the initial skeleton in 72937203dd3b.

This should fix the leak sanitizer failure discovered by
https://lab.llvm.org/buildbot/#/builders/52/builds/619.

This patch moves branch condition creation to enter the scalar epilogue
loop to VPlan. Modeling the branch in the middle block also requires
modeling the successor blocks. This is done using the recently
introduced VPIRBasicBlock.

Note that the middle.block is still created as part of the skeleton and
then patched in during VPlan execution. Unfortunately the skeleton needs
to create the middle.block early on, as it is also used for induction
resume value creation and is also needed to properly update the
dominator tree during skeleton creation.

After this patch lands, I plan to move induction resume value and phi
node creation in the scalar preheader to VPlan. Once that is done, we
should be able to create the middle.block in VPlan directly.

This is a re-worked version based on the earlier
https://reviews.llvm.org/D150398 and the main change is the use of
VPIRBasicBlock.

Depends on https://github.com/llvm/llvm-project/pull/92525

PR: https://github.com/llvm/llvm-project/pull/92651