[DAG] canCreateUndefOrPoison - only compute extract/index vector elt index knownbits when not poison

We were calling computeKnownBits to determine the bounds of the element index without ensuring that it wasn't poison, meaning if we did freeze the index, isGuaranteedNotToBeUndefOrPoison would then fail as we can't call computeKnownBits through FREEZE for potentially poison values.

Fixes #92569

Author: RKSimon

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -5246,8 +5246,12 @@ bool SelectionDAG::canCreateUndefOrPoison(SDValue Op, const APInt &DemandedElts,
     // Ensure that the element index is in bounds.
     EVT VecVT = Op.getOperand(0).getValueType();
     SDValue Idx = Op.getOperand(Opcode == ISD::INSERT_VECTOR_ELT ? 2 : 1);
-    KnownBits KnownIdx = computeKnownBits(Idx, Depth + 1);
-    return KnownIdx.getMaxValue().uge(VecVT.getVectorMinNumElements());
+    if (isGuaranteedNotToBeUndefOrPoison(Idx, DemandedElts, PoisonOnly,
+                                         Depth + 1)) {
+      KnownBits KnownIdx = computeKnownBits(Idx, Depth + 1);
+      return KnownIdx.getMaxValue().uge(VecVT.getVectorMinNumElements());
+    }
+    return true;
   }
 
   case ISD::VECTOR_SHUFFLE: {

llvm/test/CodeGen/X86/pr92569.ll

@@ -0,0 +1,29 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 4
+; RUN: llc < %s -mtriple=x86_64-linux-gnu | FileCheck %s
+
+define void @PR92569(i64 %arg, <8 x i8> %arg1) {
+; CHECK-LABEL: PR92569:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    testq %rdi, %rdi
+; CHECK-NEXT:    je .LBB0_1
+; CHECK-NEXT:  # %bb.2: # %cond.false
+; CHECK-NEXT:    rep bsfq %rdi, %rax
+; CHECK-NEXT:    jmp .LBB0_3
+; CHECK-NEXT:  .LBB0_1:
+; CHECK-NEXT:    movl $64, %eax
+; CHECK-NEXT:  .LBB0_3: # %cond.end
+; CHECK-NEXT:    shrb $3, %al
+; CHECK-NEXT:    movaps %xmm0, -{{[0-9]+}}(%rsp)
+; CHECK-NEXT:    movzbl %al, %eax
+; CHECK-NEXT:    movzbl -24(%rsp,%rax), %eax
+; CHECK-NEXT:    movl %eax, 0
+; CHECK-NEXT:    retq
+  %cttz = call i64 @llvm.cttz.i64(i64 %arg, i1 false)
+  %trunc = trunc i64 %cttz to i8
+  %lshr = lshr i8 %trunc, 3
+  %extractelement = extractelement <8 x i8> %arg1, i8 %lshr
+  %freeze = freeze i8 %extractelement
+  %zext = zext i8 %freeze to i32
+  store i32 %zext, ptr addrspace(1) null, align 4
+  ret void
+}