[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 f0b57d8)

Author: nikic

llvm/lib/Analysis/MemorySSA.cpp

@@ -2298,7 +2298,10 @@ bool MemorySSAWrapperPass::runOnFunction(Function &F) {
   return false;
 }
 
-void MemorySSAWrapperPass::verifyAnalysis() const { MSSA->verifyMemorySSA(); }
+void MemorySSAWrapperPass::verifyAnalysis() const {
+  if (VerifyMemorySSA)
+    MSSA->verifyMemorySSA();
+}
 
 void MemorySSAWrapperPass::print(raw_ostream &OS, const Module *M) const {
   MSSA->print(OS);