[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.)

llvm-svn: 228418