Do not form poison in zero reduction

Author: arsenm

llvm/test/tools/llvm-reduce/reduce-operands-target-ext-ty.ll

@@ -6,8 +6,9 @@
 
 declare void @uses_ext_ty(target("sometarget.sometype"))
 
+; TODO: Should support reduce to poison
 ; CHECK-LABEL: @foo(
-; ZERO: call void @uses_ext_ty(target("sometarget.sometype") poison)
+; ZERO: call void @uses_ext_ty(target("sometarget.sometype") %arg)
 ; ONE: call void @uses_ext_ty(target("sometarget.sometype") %arg)
 define void @foo(target("sometarget.sometype") %arg) {
   call void @uses_ext_ty(target("sometarget.sometype") %arg)

llvm/tools/llvm-reduce/deltas/ReduceOperands.cpp

@@ -140,7 +140,9 @@ void llvm::reduceOperandsZeroDeltaPass(TestRunner &Test) {
         return nullptr;
       if (TET->hasProperty(TargetExtType::HasZeroInit))
         return ConstantTargetNone::get(TET);
-      return PoisonValue::get(TET);
+
+      // TODO: Poison reduction for this case
+      return nullptr;
     }
 
     // Don't replace existing zeroes.