[VPlan] Add VPPartialDominatorTree via optional NodeTrait on DominatorTreeBase

[Copilot]

DominatorTreeBase assumed node traits were always DomTreeNodeTraits<NodeT>, making it impossible to build a VPlan dominator tree rooted at an arbitrary VPBlockBase (e.g. a region's inner header) without the tree including the entire VPlan from its top-level entry.

Changes

GenericDomTree.h — optional NodeTrait template parameter

• Added 3rd template parameter typename NodeTrait = DomTreeNodeTraits<NodeT> to DominatorTreeBase and its forward declaration, decoupling trait selection from NodeT.
• Changed the pointer-type assertion to check NodePtr sourced from the trait instead of GraphTraits<NodeT*>::NodeRef.
• findNearestCommonDominator now resolves the entry via stored Parent rather than NodeTrait::getParent(A), avoiding type mismatches when ParentPtr differs from the default.
• Relaxed getParent-based debug assertions to accept nullptr as an explicit opt-out sentinel.
• Updated out-of-line dominates/properlyDominates definitions and DomTreeNodeBase friend declarations for the 3-parameter template.

GenericSSAContext.h

• Replaced the now-stale 2-parameter forward declaration of DominatorTreeBase with a full include of GenericDomTree.h.

VPlanDominatorTree.h — partial VPlan dominator tree

• VPBlockSubgraph — lightweight struct holding a custom entry VPBlockBase *, used as the parent context.
• GraphTraits<VPBlockSubgraph*> — DFS traversal from Entry via VPAllSuccessorsIterator (region exits followed transparently).
• VPPartialDomTreeTraits — DomTreeNodeTraits-compatible struct with ParentPtr = VPBlockSubgraph*; getParent returns nullptr to opt out of the parent-equality assertion.
• VPPartialDominatorTree — DominatorTreeBase<VPBlockBase, false, VPPartialDomTreeTraits> accepting any VPBlockBase * as root:

// Full-plan tree — always rooted at Plan.getEntry()
VPDominatorTree FullDT(Plan);

// Partial tree — rooted at the region's inner header, not the VPlan entry
VPPartialDominatorTree PartialDT(Region->getEntry());
EXPECT_EQ(PartialDT.getNode(Plan.getEntry()), nullptr); // predecessor excluded
EXPECT_TRUE(PartialDT.dominates(Region->getEntry(), InnerBlock));

VPDomTreeTest.cpp — unit tests

• PartialDominanceNoRegionsTest: diamond-shaped graph rooted at a non-entry block; verifies predecessor exclusion, dominance, NCD queries, and tree-children structure. Also constructs a second VPPartialDominatorTree rooted at VPBB3 and asserts VPBB3 dominates VPBB4.
• PartialDominanceInsideRegionTest: entry inside a VPRegionBlock; confirms region block and its predecessor are absent, and that VPAllSuccessorsIterator naturally includes post-region successors.
• PartialDominanceWithCycleTest: loop with a back-edge starting from the loop header.

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