[LoopVectorizer] Restructurize code for emitting RT checks. NFCI.

[LoopVectorizer] Restructurize code for emitting RT checks. NFCI.

Place all code corresponding to a run-time check in one place.
Previously we generated some code, then proceeded to a next check, then
finished the code for the first check (like splitting blocks and
generating branches). Now the code for generating a check is
self-contained.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241741 91177308-0d34-0410-b5e6-96231b3b80d8