[NFC] Introduce surgical invalidation of IPT

[mkazantsev]

Summary

Once OrderedInstructions is able to auto-update with D51664, it makes us
much easier to invalidate Instruction Precedence Tracking: we only need to
drop cached info when we may introduce or remove the first implicit control
flow instruction.

This patch introduces a method that allows surgical invalidation of cached info
in LoopSafetyInfo only when it is needed.

[mkazantsev]

Fixed formatting.

include/llvm/Analysis/InstructionPrecedenceTracking.h

   virtual ~InstructionPrecedenceTracking() = default;
 
 public:
-  /// Clears cached information about this particular block.
+  /// Clears cached information about this particular block. We need to do it
+  /// whenever we plan to modify blocks contents in a way that may change the
+  /// first special instruction in the block.
   void invalidateBlock(const BasicBlock *BB);
 
+  /// Inform the tracking that we are going to remove or modify the instuction
+  /// \p Insn. It will do the required invalidation if needed.
+  void invalidateInstruction(const Instruction *Insn);
+
   /// Invalidates all information from this tracking.
   void clear();
 };

lib/Analysis/InstructionPrecedenceTracking.cpp

   FirstSpecialInsts.erase(BB);
 }
 
+void
+InstructionPrecedenceTracking::invalidateInstruction(const Instruction *Insn) {
+  // If we keep track of Insn, drop the cached info for it.
+  if (!isSpecialInstruction(Insn))
+    return;
+
+  auto It = FirstSpecialInsts.find(Insn->getParent());
+  if (It == FirstSpecialInsts.end())
+    return;
+
+  if (It->second == Insn)
+    FirstSpecialInsts.erase(It);
+}
+
 void InstructionPrecedenceTracking::clear() {
   FirstSpecialInsts.clear();
 #ifndef NDEBUG