JIT: Merge all RETURN/THROW blocks (#128515)

Fix #128514

`tailMergePreds(nullptr)` was called once, but my understanding is it
needs to be called repeatedly as it only processes one set at at time.

Author: BoyBaykiller

src/coreclr/jit/fgopt.cpp

@@ -5492,13 +5492,21 @@ PhaseStatus Compiler::fgHeadTailMerge(bool early)
         }
     }
 
-    predInfo.Reset();
-    for (BasicBlock* const block : retOrThrowBlocks.BottomUpOrder())
+    JITDUMP("Trying tail merge of return and throw blocks\n");
+    do
     {
-        predInfo.Push(PredInfo(block, block->lastStmt()));
-    }
-
-    tailMergePreds(nullptr);
+        predInfo.Reset();
+        for (BasicBlock* const block : retOrThrowBlocks.BottomUpOrder())
+        {
+            // If this block was already merged, skip it
+            //
+            if (!block->KindIs(BBJ_RETURN, BBJ_THROW))
+            {
+                continue;
+            }
+            predInfo.Push(PredInfo(block, block->lastStmt()));
+        }
+    } while (tailMergePreds(nullptr));
 
     // Work through any retries
     //

src/coreclr/jit/jitstd/vector.h

@@ -613,13 +613,15 @@ vector<T, Allocator>& vector<T, Allocator>::operator=(vector<T, Allocator>&& vec
 template <typename T, typename Allocator>
 typename vector<T, Allocator>::reference vector<T, Allocator>::operator[](size_type n)
 {
+    assert(n >= 0 && n < m_nSize);
     return m_pArray[n];
 }
 
 template <typename T, typename Allocator>
 typename vector<T, Allocator>::const_reference
     vector<T, Allocator>::operator[](size_type n) const
 {
+    assert(n >= 0 && n < m_nSize);
     return m_pArray[n];
 }