JIT: support SSA-aware PHI jump threading (#126812)

Teach redundant branch elimination to keep jump threading through PHI-
based blocks when the PHI uses can be fully accounted for in the block
and its immediate successors. Rewrite the affected successor SSA/VN
uses, keep dominating-block threading conservative, and add focused
regression coverage for the new PHI-based cases.

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: AndyAyersMS

src/coreclr/jit/compiler.h

@@ -7857,14 +7857,23 @@ class Compiler
 
     // Redundant branch opts
     //
-    PhaseStatus optRedundantBranches();
-    bool        optRedundantRelop(BasicBlock* const block);
-    bool        optRedundantDominatingBranch(BasicBlock* const block);
-    bool        optRedundantBranch(BasicBlock* const block);
-    bool        optJumpThreadDom(BasicBlock* const block, BasicBlock* const domBlock, bool domIsSameRelop);
-    bool        optJumpThreadPhi(BasicBlock* const block, GenTree* tree, ValueNum treeNormVN);
-    bool        optJumpThreadCheck(BasicBlock* const block, BasicBlock* const domBlock);
-    bool        optJumpThreadCore(JumpThreadInfo& jti);
+    enum class JumpThreadCheckResult
+    {
+        CannotThread,
+        CanThread,
+        NeedsPhiUseResolution,
+    };
+
+    PhaseStatus           optRedundantBranches();
+    bool                  optRedundantRelop(BasicBlock* const block);
+    bool                  optRedundantDominatingBranch(BasicBlock* const block);
+    bool                  optRedundantBranch(BasicBlock* const block);
+    bool                  optJumpThreadDom(BasicBlock* const block, BasicBlock* const domBlock, bool domIsSameRelop);
+    bool                  optJumpThreadPhi(BasicBlock* const block, GenTree* tree, ValueNum treeNormVN);
+    JumpThreadCheckResult optJumpThreadCheck(BasicBlock* const block, BasicBlock* const domBlock);
+    bool optFindPhiUsesInBlockAndSuccessors(BasicBlock* block, GenTreeLclVar* phiDef, JumpThreadInfo& jti);
+    bool optCanRewritePhiUses(JumpThreadInfo& jti);
+    bool optJumpThreadCore(JumpThreadInfo& jti);
 
     enum class ReachabilityResult
     {

src/coreclr/jit/compmemkind.h

@@ -28,6 +28,7 @@ CompMemKindMacro(LSRA)
 CompMemKindMacro(LSRA_Interval)
 CompMemKindMacro(LSRA_RefPosition)
 CompMemKindMacro(Reachability)
+CompMemKindMacro(RedundantBranch)
 CompMemKindMacro(SSA)
 CompMemKindMacro(ValueNumber)
 CompMemKindMacro(LvaTable)