[LoopInterchange] Loops with empty dependency matrix are safe.

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@326077 91177308-0d34-0410-b5e6-96231b3b80d8