[IndVars] Split loop predication out of optimizeLoopExits [NFC]

[IndVars] Split loop predication out of optimizeLoopExits [NFC]

In the process of writing D69009, I realized we have two distinct sets of invariants within this single function, and basically no shared logic.  The optimize loop exit transforms (including the new one in D69009) only care about *analyzeable* exits.  Loop predication, on the other hand, has to reason about *all* exits.  At the moment, we have the property (due to the requirement for an exact btc) that all exits are analyzeable, but that will likely change in the future as we add widenable condition support.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375138 91177308-0d34-0410-b5e6-96231b3b80d8