Add support for tail duplication to BranchFolding, and extend

tail merging support to handle more cases.
 - Recognize several cases where tail merging is beneficial even when
   the tail size is smaller than the generic threshold.
 - Make use of MachineInstrDesc::isBarrier to help detect
   non-fallthrough blocks.
 - Check for and avoid disrupting fall-through edges in more cases.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@86871 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/MachineBasicBlock.cpp

@@ -371,10 +371,7 @@ bool MachineBasicBlock::CorrectExtraCFGEdges(MachineBasicBlock *DestA,
   MachineBasicBlock::succ_iterator SI = succ_begin();
   MachineBasicBlock *OrigDestA = DestA, *OrigDestB = DestB;
   while (SI != succ_end()) {
-    if (*SI == DestA && DestA == DestB) {
-      DestA = DestB = 0;
-      ++SI;
-    } else if (*SI == DestA) {
+    if (*SI == DestA) {
       DestA = 0;
       ++SI;
     } else if (*SI == DestB) {