[LoopGuardWidening] Split out a loop pass version of GuardWidening

The idea is to have a pass which performs the same transformation as GuardWidening, but can be run within a loop pass manager without disrupting the pass manager structure.  As demonstrated by the test case, this doesn't quite get there because of issues with post dom, but it gives a good step in the right direction.  the motivation is purely to reduce compile time since we can now preserve locality during the loop walk.

This patch only includes a legacy pass.  A follow up will add a new style pass as well.

llvm-svn: 331060