[ValueTracking] Check for ConstantExpr before using recursive helpers.

[fhahn]

Summary

Make sure we do not call
constainsConstantExpression/containsUndefElement on ConstantExpression,
which is not supported.

In particular, containsUndefElement/constainsConstantExpression are only
supported on constants which are supported by getAggregateElement.

Unfortunately there's no convenient way to check if a constant supports
getAggregateElement, so just check for non-constantexpressions with
vector type. Other users of those functions do so too.

[jdoerfert]

LGTM

llvm/lib/Analysis/ValueTracking.cpp

bool llvm::isGuaranteedNotToBeUndefOrPoison(const Value *V,
   if (auto *C = dyn_cast<Constant>(V)) {
     if (isa<UndefValue>(C))
       return false;
 
     if (isa<ConstantInt>(C) || isa<GlobalVariable>(C) || isa<ConstantFP>(V) ||
         isa<ConstantPointerNull>(C) || isa<Function>(C))
       return true;
 
-    if (C->getType()->isVectorTy())
-      return !C->containsUndefElement() && !C->containsConstantExpression();
+    if (C->getType()->isVectorTy() && !isa<ConstantExpr>(C))
+      return !C->containsConstantExpression() && !C->containsUndefElement();
   }
 
   // Strip cast operations from a pointer value.
   // Note that stripPointerCastsSameRepresentation can strip off getelementptr
   // inbounds with zero offset. To guarantee that the result isn't poison, the
   // stripped pointer is checked as it has to be pointing into an allocated
   // object or be null `null` to ensure `inbounds` getelement pointers with a
   // zero offset could not produce poison.

llvm/test/Transforms/GVN/constexpr-vector-constainsundef-crash.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -gvn -S %s | FileCheck %s
+
+; Reduced test case from
+; https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=24278
+
+; Make sure we do not crash when dealing with a vector constant expression.
+define <4 x i64*> @test(i64* %ptr) {
+; CHECK-LABEL: @test(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    ret <4 x i64*> getelementptr (i64, i64* null, <4 x i64> <i64 0, i64 0, i64 0, i64 -128>)
+;
+entry:
+  %B9 = sdiv i16 -32768, 256
+  %L3 = load i64, i64* %ptr, align 4
+  %B3 = sub i16 0, %B9
+  %0 = insertelement <4 x i16> undef, i16 %B3, i32 3
+  %1 = sub <4 x i16> zeroinitializer, %0
+  %2 = sext <4 x i16> %1 to <4 x i32>
+  %3 = getelementptr inbounds i64, i64* null, <4 x i32> %2
+  %I6 = insertelement <4 x i64*> %3, i64* undef, i64 %L3
+  ret <4 x i64*> %I6
+}