[MemorySSA] Don't verify MemorySSA unless VerifyMemorySSA enabled

MemorySSA is often taking up an unreasonable fraction of runtime in
assertion enabled builds. Turns out that there is one code-path that
runs verifyMemorySSA() even if VerifyMemorySSA is not enabled. This
patch makes it conditional as well.

Differential Revision: https://reviews.llvm.org/D74505

(cherry picked from commit f0b57d80)