[NFC] ReachableBlocks: Rename isReachable method.

It was previously `isVisited`.  And mark `visit` private, it 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 visited.contains(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
@@ -55,9 +62,6 @@ class ReachableBlocks {
   ///
   /// Returns true if all reachable blocks were visited.
   bool visit(function_ref<bool(SILBasicBlock *)> visitor);
-
-  /// Return true if \p bb has been visited.
-  bool isVisited(SILBasicBlock *bb) const { return visited.contains(bb); }
 };
 
 /// Computes the set of blocks from which a path to the return-block exists.

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;
     }