[InstCombine] Add !noundef to match behavior of violating assume

[StephenFan]

Summary

The behaviors of violating assume instruction or !nonnull metadata is
different. The former is immediate undefined behavior, but the latter is
returning poison value. This patch adds !noundef to trigger immediate
undefined behavior if !nonnull is violated.

[nikic]

LGTM

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

case Intrinsic::assume: {
     CmpInst::Predicate Pred;
     Instruction *LHS;
     if (match(IIOperand, m_ICmp(Pred, m_Instruction(LHS), m_Zero())) &&
         Pred == ICmpInst::ICMP_NE && LHS->getOpcode() == Instruction::Load &&
         LHS->getType()->isPointerTy() &&
         isValidAssumeForContext(II, LHS, &DT)) {
       MDNode *MD = MDNode::get(II->getContext(), std::nullopt);
       LHS->setMetadata(LLVMContext::MD_nonnull, MD);
+      LHS->setMetadata(LLVMContext::MD_noundef, MD);
       return RemoveConditionFromAssume(II);
 
       // TODO: apply nonnull return attributes to calls and invokes
       // TODO: apply range metadata for range check patterns?
     }
 
     // Separate storage assumptions apply to the underlying allocations, not any
     // particular pointer within them. When evaluating the hints for AA purposes

llvm/test/Transforms/InstCombine/assume.ll

 ;
   tail call void @llvm.assume(i1 true) ["ignore"(ptr undef)]
   %load = load i32, ptr %P
   ret i32 %load
 }
 
 define i1 @nonnull1(ptr %a) {
 ; CHECK-LABEL: @nonnull1(
-; CHECK-NEXT:    [[LOAD:%.*]] = load ptr, ptr [[A:%.*]], align 8, !nonnull !6
+; CHECK-NEXT:    [[LOAD:%.*]] = load ptr, ptr [[A:%.*]], align 8, !nonnull !6, !noundef !6
 ; CHECK-NEXT:    tail call void @escape(ptr nonnull [[LOAD]])
 ; CHECK-NEXT:    ret i1 false
 ;
   %load = load ptr, ptr %a
   %cmp = icmp ne ptr %load, null
   tail call void @llvm.assume(i1 %cmp)
   tail call void @escape(ptr %load)
   %rval = icmp eq ptr %load, null