Dead code elimination: Update dependences after eliminating code

Without updating dependences we may lose implicit transitive dependences for
which all explicit dependences have gone through the statement iterations we
have just eliminated.

No test case. We should probably implement a -verify-dependences option.

This fixes llvm.org/PR21227

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@224459 91177308-0d34-0410-b5e6-96231b3b80d8