[X86] SimplifyMultipleUseDemandedBits - target shuffles might not be identity

If we don't demand any non-undef shuffle elements then the assert will fail as all shuffle inputs would still be flagged as 'identity' safe.

Exposed by an incoming patch.

llvm-svn: 368022

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -34758,12 +34758,13 @@ SDValue X86TargetLowering::SimplifyMultipleUseDemandedBitsForTargetNode(
         if (IdentityOp == 0)
           break;
       }
-      assert((IdentityOp == 0 || IdentityOp.countPopulation() == 1) &&
-             "Multiple identity shuffles detected");
 
       if (AllUndef)
         return DAG.getUNDEF(VT);
 
+      assert((IdentityOp == 0 || IdentityOp.countPopulation() == 1) &&
+             "Multiple identity shuffles detected");
+
       for (int i = 0; i != NumOps; ++i)
         if (IdentityOp[i])
           return DAG.getBitcast(VT, ShuffleOps[i]);