[ValueTracking] Make undef element check more precise

If we're only checking for undef, then also only look for undef
elements in the vector (rather than undef and poison).

Author: nikic

llvm/lib/Analysis/ValueTracking.cpp

@@ -7296,10 +7296,13 @@ static bool isGuaranteedNotToBeUndefOrPoison(
         isa<ConstantPointerNull>(C) || isa<Function>(C))
       return true;
 
-    if (C->getType()->isVectorTy() && !isa<ConstantExpr>(C))
-      return (!includesUndef(Kind) ? !C->containsPoisonElement()
-                                   : !C->containsUndefOrPoisonElement()) &&
-             !C->containsConstantExpression();
+    if (C->getType()->isVectorTy() && !isa<ConstantExpr>(C)) {
+      if (includesUndef(Kind) && C->containsUndefElement())
+        return false;
+      if (includesPoison(Kind) && C->containsPoisonElement())
+        return false;
+      return !C->containsConstantExpression();
+    }
   }
 
   // Strip cast operations from a pointer value.

llvm/test/Transforms/InstCombine/select.ll

@@ -2787,8 +2787,7 @@ define <2 x i8> @select_replacement_add_eq_vec_nonuniform(<2 x i8> %x, <2 x i8>
 define <2 x i8> @select_replacement_add_eq_vec_poison(<2 x i8> %x, <2 x i8> %y) {
 ; CHECK-LABEL: @select_replacement_add_eq_vec_poison(
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp eq <2 x i8> [[X:%.*]], <i8 1, i8 poison>
-; CHECK-NEXT:    [[ADD:%.*]] = add <2 x i8> [[X]], <i8 1, i8 1>
-; CHECK-NEXT:    [[SEL:%.*]] = select <2 x i1> [[CMP]], <2 x i8> [[ADD]], <2 x i8> [[Y:%.*]]
+; CHECK-NEXT:    [[SEL:%.*]] = select <2 x i1> [[CMP]], <2 x i8> <i8 2, i8 poison>, <2 x i8> [[Y:%.*]]
 ; CHECK-NEXT:    ret <2 x i8> [[SEL]]
 ;
   %cmp = icmp eq <2 x i8> %x, <i8 1, i8 poison>