[SIL] Add utils to visit prev/next insts.

For instructions at the back of a block, visiting the subsequent
instructions means visiting the instructions at the front of every
successor.  For instructions at the front of a block, visiting the prior
instructions means visiting the instructions at the back of every
predecessor.

Author: nate-chandler

include/swift/SIL/SILBasicBlock.h

@@ -742,6 +742,29 @@ inline SILFunction *SILInstruction::getFunction() const {
   return getParent()->getParent();
 }
 
+inline bool SILInstruction::visitPriorInstructions(
+    llvm::function_ref<bool(SILInstruction *)> visitor) {
+  if (auto *previous = getPreviousInstruction()) {
+    return visitor(previous);
+  }
+  for (auto *predecessor : getParent()->getPredecessorBlocks()) {
+    if (!visitor(&predecessor->back()))
+      return false;
+  }
+  return true;
+}
+inline bool SILInstruction::visitSubsequentInstructions(
+    llvm::function_ref<bool(SILInstruction *)> visitor) {
+  if (auto *next = getNextInstruction()) {
+    return visitor(next);
+  }
+  for (auto *successor : getParent()->getSuccessorBlocks()) {
+    if (!visitor(&successor->front()))
+      return false;
+  }
+  return true;
+}
+
 inline SILInstruction *SILInstruction::getPreviousInstruction() {
   auto pos = getIterator();
   return pos == getParent()->begin() ? nullptr : &*std::prev(pos);

include/swift/SIL/SILInstruction.h

@@ -517,6 +517,24 @@ class SILInstruction : public llvm::ilist_node<SILInstruction> {
   /// instruction in its block.
   SILInstruction *getNextInstruction();
 
+  /// Calls \p visitor with each instruction that is immediately prior.  Returns
+  /// false and stops visiting if \p visitor returns false.
+  ///
+  /// If \p this is is the first instruction in a block, then \p visitor is
+  /// called with the back of each predecessor.  In particular if \p this is
+  /// the first instruction of the entry block, \p visitor is never called.
+  bool
+  visitPriorInstructions(llvm::function_ref<bool(SILInstruction *)> visitor);
+
+  /// Calls \p visitor with each instruction that is immediately subsequent.
+  /// Returns false and stops visiting if \p visitor returns false.
+  ///
+  /// If \p this is the last instruction in a block, then \p visitor is called
+  /// with the front of each successor.  In particular if \p this is the last
+  /// instruction of a block without successors, \p visitor is never called.
+  bool visitSubsequentInstructions(
+      llvm::function_ref<bool(SILInstruction *)> visitor);
+
   /// This method unlinks 'self' from the containing basic block and deletes it.
   void eraseFromParent();