Clean up XFAILed test cases

We had a set of test cases that have been incomplete and XFAILED. This patch
completes a couple of the interesting ones and removes the ones which seem
redundant or not sufficiently reduced to be useful.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@211670 91177308-0d34-0410-b5e6-96231b3b80d8