Add LoopUtils class to gather some loop transformation support.

Add LoopUtils class to gather some loop transformation support.

This patch adds LoopUtils class to handle some loop related transformations. For now it has 2 transformations that simplifies other transformations such as loop unroll or unswitch:
 - Dedicate exit blocks: this ensure that all exit basic block
   (out-of-loop basic blocks that have a predecessor in the loop)
   have all their predecessors in the loop;
 - Loop Closed SSA (LCSSA): this ensure that all definitions in a loop are used inside the loop
   or in a phi instruction in an exit basic block.

It also adds the following capabilities:
 - Loop::IsLCSSA to test if the loop is in a LCSSA form
 - Loop::GetOrCreatePreHeaderBlock that can build a loop preheader if required;
 - New methods to allow on the fly updates of the loop descriptors.
 - New methods to allow on the fly updates of the CFG analysis.
 - Instruction::SetOperand to allow expression of the index relative to Instruction::NumOperands (to be compatible with the index returned by DefUseManager::ForEachUse)