[IR][ShuffleVector] Fix Wdangling-else warning in InstructionsTest

[lbenes]

Summary

Fix a dangling else that gcc-11 warned about. The EXPECT_EQ macro
expands to an if-else, so the whole construction contains a hidden
dangling else.

llvm/unittests/IR/InstructionsTest.cpp

         int Elt = std::get<0>(I);
         int ActualElt = std::get<0>(I);
 
-        if (Elt != -1)
+        if (Elt != -1) {
           EXPECT_EQ(Elt, ActualElt);
+        }
       }
 
       return /*Abort=*/false;