CodeGen: Allow critical edge splitting with structurized CFGs

[• tstellarAMD]

Summary

The rationale for not splitting critical edges with structurized CFGs was
that it would hurt performance. This isn't true in all cases, for example,
splitting cirtical edges to hoist code out of loops.

These sort of performance decisions should really be made by the optimization
passes.

[arsenm]

LGTM

[Eugene.Zelenko]

Looks like patch was not committed.

[arsenm]

This isn't necessary anymore since requiresStructuredCFG was disable for amdgcn

lib/CodeGen/MachineBasicBlock.cpp

bool MachineBasicBlock::canSplitCriticalEdge(
     const MachineBasicBlock *Succ) const {
   // Splitting the critical edge to a landing pad block is non-trivial. Don't do
   // it in this generic function.
   if (Succ->isEHPad())
     return false;
 
   const MachineFunction *MF = getParent();
 
-  // Performance might be harmed on HW that implements branching using exec mask
-  // where both sides of the branches are always executed.
-  if (MF->getTarget().requiresStructuredCFG())
-    return false;
-
   // We may need to update this's terminator, but we can't do that if
   // AnalyzeBranch fails. If this uses a jump table, we won't touch it.
   const TargetInstrInfo *TII = MF->getSubtarget().getInstrInfo();
   MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
   SmallVector<MachineOperand, 4> Cond;
   // AnalyzeBanch should modify this, since we did not allow modification.
   if (TII->analyzeBranch(*const_cast<MachineBasicBlock *>(this), TBB, FBB, Cond,
                          /*AllowModify*/ false))