[MemorySSA] Don't verify MemorySSA unless VerifyMemorySSA enabled

[nikic]

Summary

I noticed that 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.

For the test case I'm looking at right now:

Before 

  12.5533 ( 21.9%)   0.1048 (  2.9%)  12.6581 ( 20.7%)  12.6539 ( 20.7%)  Memory SSA #2
   6.1540 ( 10.7%)   0.2563 (  7.0%)   6.4103 ( 10.5%)   6.4076 ( 10.5%)  Function Integration/Inlining
   2.3936 (  4.2%)   0.1117 (  3.0%)   2.5053 (  4.1%)   2.5034 (  4.1%)  Global Value Numbering
   1.9745 (  3.4%)   0.0211 (  0.6%)   1.9955 (  3.3%)   1.9949 (  3.3%)  Rotate Loops
   1.6837 (  2.9%)   0.1032 (  2.8%)   1.7869 (  2.9%)   1.7851 (  2.9%)  Induction Variable Simplification
   1.6743 (  2.9%)   0.0324 (  0.9%)   1.7068 (  2.8%)   1.7053 (  2.8%)  Memory SSA #4
   1.4166 (  2.5%)   0.0391 (  1.1%)   1.4558 (  2.4%)   1.4547 (  2.4%)  Memory SSA #5
   1.3661 (  2.4%)   0.0389 (  1.1%)   1.4050 (  2.3%)   1.4037 (  2.3%)  Memory SSA #3
    
After
 
   6.6570 ( 14.5%)   0.2266 (  6.1%)   6.8836 ( 13.9%)   6.8809 ( 13.9%)  Function Integration/Inlining
   2.7801 (  6.1%)   0.1170 (  3.1%)   2.8972 (  5.8%)   2.8947 (  5.9%)  Global Value Numbering
   2.0526 (  4.5%)   0.0416 (  1.1%)   2.0942 (  4.2%)   2.0932 (  4.2%)  Rotate Loops
   1.8899 (  4.1%)   0.1305 (  3.5%)   2.0203 (  4.1%)   2.0183 (  4.1%)  Induction Variable Simplification

There is a similar report with much more extreme numbers at https://bugs.llvm.org/show_bug.cgi?id=44238 as well.

[asbirlea]

This had been intentionally left in in order to catch potential invalidations in debug builds.
However, since the overhead is overly large, putting it under EXPENSIVE_CHECKS seems the right thing to do.

llvm/lib/Analysis/MemorySSA.cpp

 
 bool MemorySSAWrapperPass::runOnFunction(Function &F) {
   auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
   auto &AA = getAnalysis<AAResultsWrapperPass>().getAAResults();
   MSSA.reset(new MemorySSA(F, &AA, &DT));
   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);
 }
 
 MemorySSAWalker::MemorySSAWalker(MemorySSA *M) : MSSA(M) {}
 
 /// Walk the use-def chains starting at \p StartingAccess and find