[GVN] Preserve MemorySSA if it is available.

Preserve MemorySSA if it is available before running GVN.

DSE with MemorySSA will run closely after GVN. If GVN and 2 other
passes preserve MemorySSA, DSE can re-use MemorySSA used by LICM
when doing LTO.

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D86534