ScopDetection: Do not detect regions with irreducible control as scops

Polly currently does not support irreducible control and it is probably not
worth supporting. This patch adds code that checks for irreducible control
and refuses regions containing irreducible control.

Polly traditionally had rather restrictive checks on the control flow structure
which would have refused irregular control, but within the last couple of months
most of the control flow restrictions have been removed. As part of this
generalization we accidentally allowed irregular control flow.

Contributed-by: Karthik Senthil and Ajith Pandel
llvm-svn: 258497