[LoopVectorize] Split out RuntimePointerCheck from LoopVectorizationLegality

RuntimePointerCheck will be used through LoopAccessAnalysis in
LoopVectorizationLegality.  Later in the patchset it will become a local class
of LoopAccessAnalysis.

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: 227747