Expose InsertPreheaderForLoop from LoopSimplify to other passes

Other passes, PPC counter-loop formation for example, also need to add loop
preheaders outside of the regular loop simplification pass. This makes
InsertPreheaderForLoop a global function so that it can be used by other
passes.

No functionality change intended.
Remove copied preheader insertion logic from PPCCTRLoops

Now that the preheader insertion logic in LoopSimplify is externally exposed,
use it, and remove the copy-and-pasted version.

No functionality change intended.