Add new constructors for LoopInfo/DominatorTree/BFI/BPI

Those new constructors make it more natural to construct an object for a function. For example, previously to build a LoopInfo for a function, we need four statements:

DominatorTree DT;
LoopInfo LI;
DT.recalculate(F);
LI.analyze(DT);

Now we only need one statement:

LoopInfo LI(DominatorTree(F));

http://reviews.llvm.org/D11274

llvm-svn: 242486