[LoopAccesses] Cache the result of canVectorizeMemory

LAA will be an on-demand analysis pass, so we need to cache the result
of the analysis.  canVectorizeMemory is renamed to analyzeLoop which
computes the result.  canVectorizeMemory becomes the query function for
the cached result.

This is part of the patchset that converts LoopAccessAnalysis into an
actual analysis pass.

llvm-svn: 229892