[NFC] ReachableBlocks: Extract compute method.

In preparation for adding more users.

Author: nate-chandler

include/swift/SILOptimizer/Utils/BasicBlockOptUtils.h

@@ -38,9 +38,14 @@ class SILLoopInfo;
 /// Compute the set of reachable blocks.
 class ReachableBlocks {
   BasicBlockSet visited;
+  bool isComputed;
 
 public:
-  ReachableBlocks(SILFunction *function) : visited(function) {}
+  ReachableBlocks(SILFunction *function)
+      : visited(function), isComputed(false) {}
+
+  /// Populate `visited` with the blocks reachable in the function.
+  void compute();
 
   /// 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

lib/SILOptimizer/Utils/BasicBlockOptUtils.cpp

@@ -41,6 +41,12 @@ bool ReachableBlocks::visit(function_ref<bool(SILBasicBlock *)> visitor) {
   return true;
 }
 
+void ReachableBlocks::compute() {
+  // Visit all the blocks without doing any extra work.
+  visit([](SILBasicBlock *) { return true; });
+  isComputed = true;
+}
+
 ReachingReturnBlocks::ReachingReturnBlocks(SILFunction *function)
     : worklist(function) {
   for (SILBasicBlock &block : *function) {
@@ -57,8 +63,7 @@ ReachingReturnBlocks::ReachingReturnBlocks(SILFunction *function)
 
 bool swift::removeUnreachableBlocks(SILFunction &f) {
   ReachableBlocks reachable(&f);
-  // Visit all the blocks without doing any extra work.
-  reachable.visit([](SILBasicBlock *) { return true; });
+  reachable.compute();
 
   // Remove the blocks we never reached. Assume the entry block is visited.
   // Reachable's visited set contains dangling pointers during this loop.