Remove unneeded RegionSimplify pass.

We now support regions with multiple entries and multiple exits natively.
Regions are not needed to be simplified to single entry and single exit.

We need to XFAIL two test cases as this change increases the scop coverage
and uncoveres two failures in the independent blocks pass. The first failure
will be fixed in a subsequent commit, the second one is in the non-default
-polly-codegen-scev mode and still needs to be fixed.

Contributed-by: Star Tan <tanmx_star@yeah.net>
llvm-svn: 179673