[LV] Preserve LAA in LoopVectorize (NFCI).
LoopVectorize already always preserves DT, LI and SCEV. If any changes
get made to the CFG, cached LAA info for loops are cleared.
LoopAccessAnalysis also implements ::invalidate to clear the analysis if
SE, DT or LI gets invalidated. Hence it should be safe to preserve LAA
and save a small amount of compile-time.
