[NewGVN] Abort PHIOfOps if singleton PHI is found

[ManuelJBrito]

Summary

Currently we just bypass singleton phis, however we know that in order to create the phi of ops all phis must be in the same block. Therefore if one phi is a singleton then the rest are as well.

[nlopes]

LGTM

llvm/lib/Transforms/Scalar/NewGVN.cpp

if (!SamePHIBlock) {
       SamePHIBlock = getBlockForValue(OpPHI);
     } else if (SamePHIBlock != getBlockForValue(OpPHI)) {
       LLVM_DEBUG(
           dbgs()
           << "PHIs for operands are not all in the same block, aborting\n");
       return nullptr;
     }
     // No point in doing this for one-operand phis.
-    if (OpPHI->getNumOperands() == 1) {
-      OpPHI = nullptr;
-      continue;
-    }
+    // Since all PHIs for operands must be in the same block, then they must
+    // have the same number of operands so we can just abort.
+    if (OpPHI->getNumOperands() == 1)
+      return nullptr;
   }
 
   if (!OpPHI)
     return nullptr;
 
   SmallVector<ValPair, 4> PHIOps;
   SmallPtrSet<Value *, 4> Deps;
   auto *PHIBlock = getBlockForValue(OpPHI);