[LoopVectorize] Move RuntimePointerCheck under LoopAccessAnalysis

This class needs to remain public because it's used by
LoopVectorizationLegality::addRuntimeCheck.

NFC.  This is part of the patchset that splits out the memory dependence logic
from LoopVectorizationLegality into a new class LoopAccessAnalysis.
LoopAccessAnalysis will be used by the new Loop Distribution pass.

llvm-svn: 227755