[NFC] Remove obsolete checks followed by fix of isGuaranteedToTransferExecutionToSuccessor

[mkazantsev]

Summary

In past, isGuaranteedToTransferExecutionToSuccessor contained some weird logic
for volatile loads/stores that was ultimately removed by patch D65375. It's time to
remove a logic in other place that was supposed to work around about the deleted
logic.

[uenoku]

This makes sense to me.

llvm/lib/Analysis/InstructionPrecedenceTracking.cpp

 
 bool ImplicitControlFlowTracking::isSpecialInstruction(
     const Instruction *Insn) const {
   // If a block's instruction doesn't always pass the control to its successor
   // instruction, mark the block as having implicit control flow. We use them
   // to avoid wrong assumptions of sort "if A is executed and B post-dominates
   // A, then B is also executed". This is not true is there is an implicit
   // control flow instruction (e.g. a guard) between them.
-  //
-  // TODO: Currently, isGuaranteedToTransferExecutionToSuccessor returns false
-  // for volatile stores and loads because they can trap. The discussion on
-  // whether or not it is correct is still ongoing. We might want to get rid
-  // of this logic in the future. Anyways, trapping instructions shouldn't
-  // introduce implicit control flow, so we explicitly allow them here. This
-  // must be removed once isGuaranteedToTransferExecutionToSuccessor is fixed.
-  if (isGuaranteedToTransferExecutionToSuccessor(Insn))
-    return false;
-  if (isa<LoadInst>(Insn)) {
-    assert(cast<LoadInst>(Insn)->isVolatile() &&
-           "Non-volatile load should transfer execution to successor!");
-    return false;
-  }
-  if (isa<StoreInst>(Insn)) {
-    assert(cast<StoreInst>(Insn)->isVolatile() &&
-           "Non-volatile store should transfer execution to successor!");
-    return false;
-  }
-  return true;
+  return !isGuaranteedToTransferExecutionToSuccessor(Insn);
 }
 
 bool MemoryWriteTracking::isSpecialInstruction(
     const Instruction *Insn) const {
   using namespace PatternMatch;
   if (match(Insn, m_Intrinsic<Intrinsic::experimental_widenable_condition>()))
     return false;
   return Insn->mayWriteToMemory();
 }