[PM] Make LowerAtomic a FunctionPass

[davide]

Summary

LowerAtomic happens to be a BasicBlockPass but the scope on which it operates is actually a function.
In general, I don't think BasicBlockPasses should exist at all and can be replaced by FunctionPass(es) which iterate over the set of BasicBlocks. If this looks fine, I'll port the pass to the new PM.

[reames]

LGTM

[chandlerc]

I'm fine with this, but I don't actually think you should really aggressively port basic block passes. I'm fine if it accomplishes something specifically, but in many cases I'd just leave the old basic block pass in place and add a clean function pass for the new pass manager having them both call out to a helper function.

lib/Transforms/Scalar/LowerAtomic.cpp

124–126

Please use clang-format and skip the braces here.

lib/Transforms/Scalar/LowerAtomic.cpp

 }
 
 static bool LowerStoreInst(StoreInst *SI) {
   SI->setAtomic(AtomicOrdering::NotAtomic);
   return true;
 }
 
 namespace {
-  struct LowerAtomic : public BasicBlockPass {
+  struct LowerAtomic : public FunctionPass {
     static char ID;
-    LowerAtomic() : BasicBlockPass(ID) {
+
+    LowerAtomic() : FunctionPass(ID) {
       initializeLowerAtomicPass(*PassRegistry::getPassRegistry());
     }
-    bool runOnBasicBlock(BasicBlock &BB) override {
-      if (skipBasicBlock(BB))
+
+    bool runOnFunction(Function &F) override {
+      if (skipFunction(F))
         return false;
       bool Changed = false;
-      for (BasicBlock::iterator DI = BB.begin(), DE = BB.end(); DI != DE; ) {
+      for (BasicBlock &BB: F) {
+        Changed |= runOnBasicBlock(BB);
+      }
+      return Changed;
+    }
+
+  private:
+    bool runOnBasicBlock(BasicBlock &BB) {
+      bool Changed = false;
+      for (BasicBlock::iterator DI = BB.begin(), DE = BB.end(); DI != DE;) {
         Instruction *Inst = &*DI++;
         if (FenceInst *FI = dyn_cast<FenceInst>(Inst))
           Changed |= LowerFenceInst(FI);
         else if (AtomicCmpXchgInst *CXI = dyn_cast<AtomicCmpXchgInst>(Inst))
           Changed |= LowerAtomicCmpXchgInst(CXI);
         else if (AtomicRMWInst *RMWI = dyn_cast<AtomicRMWInst>(Inst))
           Changed |= LowerAtomicRMWInst(RMWI);
         else if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) {