LoopLoadElimination, LoopVersioning and LoopVectorize currently
fetch MemorySSA when construction LoopAccessAnalysis. However,
LoopAccessAnalysis does not actually use MemorySSA and we can pass
nullptr instead.

This saves one MemorySSA calculation in the default pipeline, and
thus improves compile-time.

Differential Revision: https://reviews.llvm.org/D108074