[LoopInterchange] Loops with empty dependency matrix are safe.

The dependency matrix is only empty if no conflicting load/store
instructions have been found. In that case, it is safe to interchange.

For the LLVM test-suite, after this change around 1900 loops are
interchanged, whereas it is 15 before this change. On cortex-a57,
this gives an improvement of -0.57% on the geomean execution
time of SPEC2006, SPEC2000 and the test-suite. There are a
few small perf regressions, but I think we can improve on those
by making the cost model better.

Reviewers: karthikthecool, mcrosier

Reviewed by: karthikthecool

Differential Revision: https://reviews.llvm.org/D43236

llvm-svn: 326077

lib/Transforms/Scalar/LoopInterchange.cpp

@@ -173,9 +173,6 @@ static bool populateDependencyMatrix(CharMatrix &DepMatrix, unsigned Level,
     }
   }
 
-  // We don't have a DepMatrix to check legality return false.
-  if (DepMatrix.empty())
-    return false;
   return true;
 }