[Scalar] Fix a warning

This patch fixes:

  llvm/lib/Transforms/Scalar/LowerMatrixIntrinsics.cpp:628:22: error:
  variable 'Inst' set but not used [-Werror,-Wunused-but-set-variable]

Author: kazutakahirata

llvm/lib/Transforms/Scalar/LowerMatrixIntrinsics.cpp

@@ -641,6 +641,7 @@ class LowerMatrixIntrinsics {
             << " shuffles beacuse we do not have a shape-aware lowering for "
                "its def:\n"
             << *Inst << '\n');
+        (void)Inst;
         SplitMatrices++;
       } else {
         // The ShapeMap has it, so it's a case where we're being lowered