[LV] Don't bail to MiddleBlock if a runtime check fails, bail to ScalarPH instead

We were bailing to two places if our runtime checks failed. If the initial overflow check failed, we'd go to ScalarPH. If any other check failed, we'd go to MiddleBlock. This caused us to have to have an extra PHI per induction and reduction as the vector loop's exit block was not dominated by its latch.

There's no need to have this behavior - if we just always go to ScalarPH we can get rid of a bunch of complexity.

llvm-svn: 246637