[LoopGuardWidening] Make PostDomTree optional

The effect of doing so is not disrupting the LoopPassManager when mixing this pass with other loop passes.  This should help locality of access substaintially and avoids the cost of computing PostDom.

The assumption here is that the full GuardWidening (which does use PostDom) is run as a canonicalization before loop opts and that this version is just catching cases exposed by other loop passes.  (i.e. LoopPredication, IndVarSimplify, LoopUnswitch, etc..)

llvm-svn: 331094