Dead code elimination: Schedule another approximative step before actual DCE

In 'obsequi' we have a scop in which the current dead code elimination works,
but the generated code is way too complex. To avoid this trouble (and to not
disable the DCE entirely) we add an additional approximative step before
the actual dead code elimination. This should fix one of the two current
nightly-test issues.

Polly could be improved to handle 'obsequi' by teaching it to introduce only a
single parameter for (%1 and zext %1) which halves the number of parameters and
allows polly to derive a simpler representation for the set of live iterations.
However, this needs some time to investigate.

I will commit a test case as soon as we have a reduced one.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@202010 91177308-0d34-0410-b5e6-96231b3b80d8