[InstSimplify] remove poison-unsafe insertelement of undef value

PR45481:
https://bugs.llvm.org/show_bug.cgi?id=45481

SDAG has an identical transform to this, so there's little
chance of any real-world impact. OTOH, that means we are
effectively sweeping the bug out of sight because poison
exists in codegen too.

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -4302,11 +4302,6 @@ Value *llvm::SimplifyInsertElementInst(Value *Vec, Value *Val, Value *Idx,
   if (isa<UndefValue>(Idx))
     return UndefValue::get(Vec->getType());
 
-  // Inserting an undef scalar? Assume it is the same value as the existing
-  // vector element.
-  if (isa<UndefValue>(Val))
-    return Vec;
-
   // If we are extracting a value from a vector, then inserting it into the same
   // place, that's the input vector:
   // insertelt Vec, (extractelt Vec, Idx), Idx --> Vec

llvm/test/Transforms/InstCombine/extractelement.ll

@@ -319,7 +319,8 @@ define <4 x double> @invalid_extractelement(<2 x double> %a, <4 x double> %b, do
 ; ANY-NEXT:    [[T4:%.*]] = shufflevector <4 x double> [[B:%.*]], <4 x double> [[TMP1]], <4 x i32> <i32 undef, i32 1, i32 4, i32 3>
 ; ANY-NEXT:    [[E:%.*]] = extractelement <4 x double> [[B]], i32 1
 ; ANY-NEXT:    store double [[E]], double* [[P:%.*]], align 8
-; ANY-NEXT:    ret <4 x double> [[T4]]
+; ANY-NEXT:    [[R:%.*]] = insertelement <4 x double> [[T4]], double undef, i64 0
+; ANY-NEXT:    ret <4 x double> [[R]]
 ;
   %t3 = extractelement <2 x double> %a, i32 0
   %t4 = insertelement <4 x double> %b, double %t3, i32 2

llvm/test/Transforms/InstSimplify/insertelement.ll

@@ -42,14 +42,26 @@ define <4 x i32> @test5(<4 x i32> %A) {
   ret <4 x i32> %I
 }
 
+; The undef may be replacing a poison value, so it is not safe to just return 'A'.
+
 define <4 x i32> @PR1286(<4 x i32> %A) {
 ; CHECK-LABEL: @PR1286(
-; CHECK-NEXT:    ret <4 x i32> [[A:%.*]]
+; CHECK-NEXT:    [[B:%.*]] = insertelement <4 x i32> [[A:%.*]], i32 undef, i32 1
+; CHECK-NEXT:    ret <4 x i32> [[B]]
 ;
   %B = insertelement <4 x i32> %A, i32 undef, i32 1
   ret <4 x i32> %B
 }
 
+define <2 x i32> @undef_into_constant_vector_with_variable_index(<2 x i32> %A, i32 %Index) {
+; CHECK-LABEL: @undef_into_constant_vector_with_variable_index(
+; CHECK-NEXT:    [[B:%.*]] = insertelement <2 x i32> <i32 42, i32 -42>, i32 undef, i32 [[INDEX:%.*]]
+; CHECK-NEXT:    ret <2 x i32> [[B]]
+;
+  %B = insertelement <2 x i32> <i32 42, i32 -42>, i32 undef, i32 %Index
+  ret <2 x i32> %B
+}
+
 define <8 x i8> @extract_insert_same_vec_and_index(<8 x i8> %in) {
 ; CHECK-LABEL: @extract_insert_same_vec_and_index(
 ; CHECK-NEXT:    ret <8 x i8> [[IN:%.*]]