don't use pred assertions from unreachables

Author: AndyAyersMS

src/coreclr/jit/compiler.h

@@ -4799,7 +4799,7 @@ class Compiler
     FoldResult fgFoldConditional(BasicBlock* block);
 
     PhaseStatus fgMorphBlocks();
-    void fgMorphBlock(BasicBlock* block);
+    void fgMorphBlock(BasicBlock* block, unsigned highestReachablePostorder = 0);
     void fgMorphStmts(BasicBlock* block);
 
     void fgMergeBlockReturn(BasicBlock* block);

src/coreclr/jit/morph.cpp

@@ -13772,8 +13772,10 @@ void Compiler::fgMorphStmts(BasicBlock* block)
 //
 // Arguments:
 //    block - block in question
+//    highestReachablePostorder - maximum postorder number for a
+//     reachable block.
 //
-void Compiler::fgMorphBlock(BasicBlock* block)
+void Compiler::fgMorphBlock(BasicBlock* block, unsigned highestReachablePostorder)
 {
     JITDUMP("\nMorphing " FMT_BB "\n", block->bbNum);
 
@@ -13788,6 +13790,8 @@ void Compiler::fgMorphBlock(BasicBlock* block)
         }
         else
         {
+            assert(highestReachablePostorder > 0);
+
             // Determine if this block can leverage assertions from its pred blocks.
             //
             // Some blocks are ineligible.
@@ -13818,6 +13822,14 @@ void Compiler::fgMorphBlock(BasicBlock* block)
                         break;
                     }
 
+                    if (pred->bbPostorderNum > highestReachablePostorder)
+                    {
+                        // This pred was not reachable from the original DFS root set, so
+                        // we can ignore its assertion information.
+                        //
+                        continue;
+                    }
+
                     // Yes, pred assertions are available. If this is the first pred, copy.
                     // If this is a subsequent pred, intersect.
                     //
@@ -13939,7 +13951,7 @@ PhaseStatus Compiler::fgMorphBlocks()
         // We are optimizing. Process in RPO.
         //
         fgRenumberBlocks();
-        fgDfsReversePostorder();
+        const unsigned highestReachablePostorder = fgDfsReversePostorder();
 
         // Disallow general creation of new blocks or edges as it
         // would invalidate RPO.
@@ -13971,7 +13983,7 @@ PhaseStatus Compiler::fgMorphBlocks()
         for (unsigned i = 1; i <= bbNumMax; i++)
         {
             BasicBlock* const block = fgBBReversePostorder[i];
-            fgMorphBlock(block);
+            fgMorphBlock(block, highestReachablePostorder);
         }
         assert(bbNumMax == fgBBNumMax);