[CFG] Replace hardcoded max BBs explored as CL option. NFC.

[anna]

Summary

This option was hardcoded to 32. Changing this as a CL option since we
have seen some cases downstream where increasing this limit allows us to
disprove reachability.

[jdoerfert]

LGTM, one nit though

llvm/lib/Analysis/CFG.cpp

164

If you initialize with DefaultMaxBBsToExplore you don't need to read the global multiple times.

[anna]

llvm/lib/Analysis/CFG.cpp

164

good point, will do. Thanks for the review Johannes!

llvm/lib/Analysis/CFG.cpp

 //===-- CFG.cpp - BasicBlock analysis --------------------------------------==//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // This family of functions performs analyses on basic blocks, and instructions
 // contained within basic blocks.
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Analysis/CFG.h"
 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/IR/Dominators.h"
+#include "llvm/Support/CommandLine.h"
 
 using namespace llvm;
 
+// The max number of basic blocks explored during reachability analysis between
+// two basic blocks. This is kept reasonably small to limit compile time when
+// repeatedly used by clients of this analysis (such as captureTracking).
+static cl::opt<unsigned> DefaultMaxBBsToExplore(
+    "dom-tree-reachability-max-bbs-to-explore", cl::Hidden,
+    cl::desc("Max number of BBs to explore for reachability analysis"),
+    cl::init(32));
+
 /// FindFunctionBackedges - Analyze the specified function to find all of the
 /// loop backedges in the function and return them.  This is a relatively cheap
 /// (compared to computing dominators and loop info) analysis.
 ///
 /// The output is added to Result, as pairs of <from,to> edge info.
 void llvm::FindFunctionBackedges(const Function &F,
      SmallVectorImpl<std::pair<const BasicBlock*,const BasicBlock*> > &Result) {
   const BasicBlock *BB = &F.getEntryBlock();
if (LI && ExclusionSet) {
     for (auto BB : *ExclusionSet) {
       if (const Loop *L = getOutermostLoop(LI, BB))
         LoopsWithHoles.insert(L);
     }
   }
 
   const Loop *StopLoop = LI ? getOutermostLoop(LI, StopBB) : nullptr;
 
-  // Limit the number of blocks we visit. The goal is to avoid run-away compile
-  // times on large CFGs without hampering sensible code. Arbitrarily chosen.
-  unsigned Limit = 32;
+  unsigned Limit = DefaultMaxBBsToExplore;
   SmallPtrSet<const BasicBlock*, 32> Visited;
   do {
     BasicBlock *BB = Worklist.pop_back_val();
     if (!Visited.insert(BB).second)
       continue;
     if (BB == StopBB)
       return true;
     if (ExclusionSet && ExclusionSet->count(BB))