LoopVectorizer: Add a check that the backedge taken count + 1 does not overflow

The loop vectorizer instantiates be-taken-count + 1 as the loop iteration count.
If this expression overflows the generated code was invalid.

In case of overflow the code now jumps to the scalar loop.

Fixes PR17288.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209854 91177308-0d34-0410-b5e6-96231b3b80d8