[MBP] Insert target hook to allow targets to decide about MBB movements

[andcarminati]

Summary

This patch includes a mechanism to give the target the final decision about moving a block up. The rationale behind this is that some targets have hardware low-overhead loop instructions that can benefit from the change, ie, loop instructions that perform better for backward targets. Some targets, like Arm have a custom block reordering pass to improve the layout, but this change simplifies the approach.

llvm/include/llvm/CodeGen/TargetInstrInfo.h

public:
   virtual void insertIndirectBranch(MachineBasicBlock &MBB,
                                     MachineBasicBlock &NewDestBB,
                                     MachineBasicBlock &RestoreBB,
                                     const DebugLoc &DL, int64_t BrOffset = 0,
                                     RegScavenger *RS = nullptr) const {
     llvm_unreachable("target did not implement");
   }
 
+  /// Analyze a basic block \p Block and its instructions and tell if it
+  /// is profitable to move it up in the final layout of a function. Some
+  /// hardware loop instructions can work better with backward branches.
+  virtual bool isProfitableToMoveUp(const MachineBasicBlock *Block) const {
+    return true;
+  }
+
   /// Analyze the branching code at the end of MBB, returning
   /// true if it cannot be understood (e.g. it's a switch dispatch or isn't
   /// implemented for a target).  Upon success, this returns false and returns
   /// with the following information in various cases:
   ///
   /// 1. If this block ends with no branches (it just falls through to its succ)
   ///    just return false, leaving TBB/FBB null.
   /// 2. If this block ends with only an unconditional branch, it sets TBB to be

llvm/lib/CodeGen/MachineBlockPlacement.cpp

 //
 // If BB is moved before OldTop, Pred needs a taken branch to BB, and it can't
 // layout the other successor below it, so it can't reduce taken branch.
 // In this case we keep its original layout.
 bool
 MachineBlockPlacement::canMoveBottomBlockToTop(
     const MachineBasicBlock *BottomBlock,
     const MachineBasicBlock *OldTop) {
+
+  if (!TII->isProfitableToMoveUp(BottomBlock))
+    return false;
+
   if (BottomBlock->pred_size() != 1)
     return true;
   MachineBasicBlock *Pred = *BottomBlock->pred_begin();
   if (Pred->succ_size() != 2)
     return true;
 
   MachineBasicBlock *OtherBB = *Pred->succ_begin();
   if (OtherBB == BottomBlock)