[ValueTracking] Teach isGuaranteedNotToBeUndefOrPoison about splats

Splats include two poison values, but only the poison-ness of the
splatted value actually matters.

Author: c-rhodes

llvm/lib/Analysis/ValueTracking.cpp

@@ -7678,6 +7678,7 @@ static bool isGuaranteedNotToBeUndefOrPoison(
         return true;
     }
 
+    Value *Splat;
     if (!::canCreateUndefOrPoison(Opr, Kind,
                                   /*ConsiderFlagsAndMetadata=*/true)) {
       if (const auto *PN = dyn_cast<PHINode>(V)) {
@@ -7695,6 +7696,10 @@ static bool isGuaranteedNotToBeUndefOrPoison(
         }
         if (IsWellDefined)
           return true;
+      } else if (isa<ShuffleVectorInst>(Opr) && (Splat = getSplatValue(Opr))) {
+        // For splats we only need to check the value being splatted.
+        if (OpCheck(Splat))
+          return true;
       } else if (all_of(Opr->operands(), OpCheck))
         return true;
     }

llvm/unittests/Analysis/ValueTrackingTest.cpp

@@ -1091,6 +1091,16 @@ TEST_F(ValueTrackingTest, isGuaranteedNotToBeUndefOrPoison) {
   }
 }
 
+TEST_F(ValueTrackingTest, isGuaranteedNotToBeUndefOrPoison_splat) {
+  parseAssembly(
+      "define <4 x i32> @test(i32 noundef %x) {\n"
+      "  %ins = insertelement <4 x i32> poison, i32 %x, i32 0\n"
+      "  %A = shufflevector <4 x i32> %ins, <4 x i32> poison, <4 x i32> zeroinitializer\n"
+      "  ret <4 x i32> %A\n"
+      "}");
+  EXPECT_TRUE(isGuaranteedNotToBeUndefOrPoison(A));
+}
+
 TEST_F(ValueTrackingTest, isGuaranteedNotToBeUndefOrPoison_assume) {
   parseAssembly("declare i1 @f_i1()\n"
                 "declare i32 @f_i32()\n"