[LoopVectorize] Add accessors for Num{Stores,Loads,PredStores} in AccessAnalysis

These members are moving to LoopAccessAnalysis.  The accessors help to hide
this.

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: 227750