[LVer] Fix FIXME: hide addPHINodes, NFC

Since Ashutosh made findDefsUsedOutsideOfLoop public, we can clean this
up.

Now clients that don't compute DefsUsedOutsideOfLoop can just call
versionLoop() and computing DefsUsedOutsideOfLoop will happen
implicitly.  With that there is no reason to expose addPHINodes anymore.

Ashutosh, you can now drop the calls to findDefsUsedOutsideOfLoop and
addPHINodes in LVerLICM and things should just work.

llvm-svn: 245579