[LV] Move addRuntimeCheck to LoopAccessAnalysis

[LV] Move addRuntimeCheck to LoopAccessAnalysis

This will allow it to be shared with the new Loop Distribution pass.

getFirstInst is currently duplicated across LoopVectorize.cpp and
LoopAccessAnalysis.cpp.  This is a short-term work-around until we figure out
a better solution.

NFC.  (The code moved is adjusted a bit for the name of the Loop member and
that PtrRtCheck is now a reference rather than a pointer.)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228418 91177308-0d34-0410-b5e6-96231b3b80d8