[LV][CFG] Add irreducible CFG detection for outer loops

This patch adds support for detecting outer loops with irreducible control
flow in LV. Current detection uses SCCs and only works for innermost loops.
This patch adds a utility function that works on any CFG, given its RPO
traversal and its LoopInfoBase. This function is a generalization
of isIrreducibleCFG  from lib/CodeGen/ShrinkWrap.cpp. The code in
lib/CodeGen/ShrinkWrap.cpp is also updated to use the new generic utility
function.

Patch by Diego Caballero <diego.caballero@intel.com>

Differential Revision: https://reviews.llvm.org/D40874

llvm-svn: 326568