AtomicExpand: Really allow incremental legalization (#108613)

Fix up 100d9b8. The iterator
fixes ended up defeating the point, since newly inserted blocks
were not visited. This never erases the current block, so we can
simply not preincrement the block iterator.

The AArch64 FP atomic tests now expand the cmpxchg in the second
round of legalization.

Author: arsenm

llvm/lib/CodeGen/AtomicExpandPass.cpp

@@ -351,9 +351,8 @@ bool AtomicExpandImpl::run(Function &F, const TargetMachine *TM) {
 
   bool MadeChange = false;
 
-  for (Function::iterator BBI = F.begin(), BBE = F.end(); BBI != BBE;) {
+  for (Function::iterator BBI = F.begin(), BBE = F.end(); BBI != BBE; ++BBI) {
     BasicBlock *BB = &*BBI;
-    ++BBI;
 
     BasicBlock::reverse_iterator Next;
 
@@ -365,14 +364,8 @@ bool AtomicExpandImpl::run(Function &F, const TargetMachine *TM) {
       if (processAtomicInstr(&Inst)) {
         MadeChange = true;
 
-        // Detect control flow change and resume iteration from the original
-        // block to inspect any newly inserted blocks. This allows incremental
-        // legalization of atomicrmw and cmpxchg.
-        if (Next != E && BB != Next->getParent()) {
-          BBI = BB->getIterator();
-          BBE = F.end();
-          break;
-        }
+        // New blocks may have been inserted.
+        BBE = F.end();
       }
     }
   }