[MemorySSA] Move code around a bit. NFC.

This patch moves MSSA's caching walker into MemorySSA, and moves the
actual definition of MSSA's caching walker out of MemorySSA.h. This is
done in preparation for the new walker, which should be out for review
soonish.

Also, this patch removes a field from UpwardsMemoryQuery and has a few
lines of diff from clang-format'ing MemorySSA.cpp.

llvm-svn: 273723