[NFC] ReachableBlocks: Add isReachable method.

Mark visit and isVisited private, these needn't be part of the interface
for any current clients.

Author: nate-chandler

include/swift/SILOptimizer/Utils/BasicBlockOptUtils.h

@@ -47,6 +47,13 @@ class ReachableBlocks {
   /// Populate `visited` with the blocks reachable in the function.
   void compute();
 
+  /// Whether `block` is reachable from the entry block.
+  bool isReachable(SILBasicBlock *block) const {
+    assert(isComputed);
+    return isVisited(block);
+  }
+
+private:
   /// Invoke \p visitor for each reachable block in \p f in worklist order (at
   /// least one predecessor has been visited--defs are always visited before
   /// uses except for phi-type block args). The \p visitor takes a block

lib/SILOptimizer/Utils/BasicBlockOptUtils.cpp

@@ -70,7 +70,7 @@ bool swift::removeUnreachableBlocks(SILFunction &f) {
   bool changed = false;
   for (auto ii = std::next(f.begin()), end = f.end(); ii != end;) {
     auto *bb = &*ii++;
-    if (!reachable.isVisited(bb)) {
+    if (!reachable.isReachable(bb)) {
       bb->removeDeadBlock();
       changed = true;
     }