[DAGCombiner] visitFREEZE: Early exit when N is deleted (llvm#128161)

`N` may get merged with existing nodes inside the loop. Early exit when
it is deleted to avoid the crash.
Alternative solution: use `DAGNodeDeletedListener` to refresh the value
of N.

Closes llvm#128143.

(cherry picked from commit 646e4f2)

Author: dtcxzyw

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -16170,11 +16170,13 @@ SDValue DAGCombiner::visitFREEZE(SDNode *N) {
       DAG.UpdateNodeOperands(FrozenMaybePoisonOperand.getNode(),
                              MaybePoisonOperand);
     }
+
+    // This node has been merged with another.
+    if (N->getOpcode() == ISD::DELETED_NODE)
+      return SDValue(N, 0);
   }
 
-  // This node has been merged with another.
-  if (N->getOpcode() == ISD::DELETED_NODE)
-    return SDValue(N, 0);
+  assert(N->getOpcode() != ISD::DELETED_NODE && "Node was deleted!");
 
   // The whole node may have been updated, so the value we were holding
   // may no longer be valid. Re-fetch the operand we're `freeze`ing.

llvm/test/CodeGen/X86/pr128143.ll

@@ -0,0 +1,32 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc < %s -mtriple=x86_64-- | FileCheck %s
+
+@g_1 = external global i8
+@g_2 = external global i8
+
+; Make sure we don't crash on this test.
+
+define i1 @test(i1 %cmp1, i32 %x) {
+; CHECK-LABEL: test:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    movq g_2@GOTPCREL(%rip), %rcx
+; CHECK-NEXT:    xorl %eax, %eax
+; CHECK-NEXT:    cmpq %rcx, g_1@GOTPCREL(%rip)
+; CHECK-NEXT:    setne %al
+; CHECK-NEXT:    cmpl %eax, %esi
+; CHECK-NEXT:    setb %cl
+; CHECK-NEXT:    orb %cl, %al
+; CHECK-NEXT:    andb %dil, %al
+; CHECK-NEXT:    # kill: def $al killed $al killed $eax
+; CHECK-NEXT:    retq
+entry:
+  %cmp2 = icmp ne ptr @g_1, @g_2
+  %fr = freeze ptr @g_1
+  %cmp3 = icmp ne ptr %fr, @g_2
+  %ext1 = zext i1 %cmp3 to i32
+  %sel1 = select i1 %cmp1, i1 %cmp2, i1 false
+  %cmp4 = icmp ult i32 %x, %ext1
+  %sel3 = select i1 %cmp1, i1 %cmp4, i1 false
+  %or = or i1 %sel1, %sel3
+  ret i1 %or
+}