[MemorySSA] Clear VisitedBlocks per query

The problem can be shown from the newly added test case.
There are two invocations to MemorySSAUpdater::moveToPlace, and the
internal data structure VisitedBlocks is changed in the first
invocation, and reused in the second invocation. In between the two
invocations, there is a change to the CFG, and MemorySSAUpdater is
notified about the change.

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D119898