[CodeGen] Make MI.isLabel() return true for LOCAL_ESCAPE.

[ab]

Summary

Found by manual inspection: LOCAL_ESCAPE seems to have the same
properties as EH/GC labels: we can't remove it, and moving it is
constrained. Make isLabel acknowledge that.

The only use of isLabel is isPosition.

isPosition causes isSafeToMove to return false, which IIUC is
conservative intra-block but seems correct across blocks.

That in turn prevents DeadMachineInstructionElim from removing the
instruction, without the need for a special case.

The other changed uses of isPosition all seem harmless at worst:

• AsmPrinter.cpp: doesn't count the label
• MachineBasicBlock.cpp: changes the PHI elimination insert pt
• MachineCSE.cpp: skips the label
• PeepholeOptimizer.cpp: skips the label
• TargetInstrInfo.cpp: scheduling boundary
• X86InstrInfo.cpp: blocks outlining

include/llvm/CodeGen/MachineInstr.h

public:
   /// Unlink 'this' form its basic block and delete it.
   ///
   /// If the instruction is part of a bundle, the other instructions in the
   /// bundle remain bundled.
   void eraseFromBundle();
 
   bool isEHLabel() const { return getOpcode() == TargetOpcode::EH_LABEL; }
   bool isGCLabel() const { return getOpcode() == TargetOpcode::GC_LABEL; }
+  bool isLocalEscape() const {
+    return getOpcode() == TargetOpcode::LOCAL_ESCAPE;
+  }
 
   /// Returns true if the MachineInstr represents a label.
-  bool isLabel() const { return isEHLabel() || isGCLabel(); }
+  bool isLabel() const {
+    return isEHLabel() || isGCLabel() || isLocalEscape();
+  }
   bool isCFIInstruction() const {
     return getOpcode() == TargetOpcode::CFI_INSTRUCTION;
   }
 
   // True if the instruction represents a position in the function.
   bool isPosition() const { return isLabel() || isCFIInstruction(); }
 
   bool isDebugValue() const { return getOpcode() == TargetOpcode::DBG_VALUE; }

lib/CodeGen/DeadMachineInstructionElim.cpp

 
 bool DeadMachineInstructionElim::isDead(const MachineInstr *MI) const {
   // Technically speaking inline asm without side effects and no defs can still
   // be deleted. But there is so much bad inline asm code out there, we should
   // let them be.
   if (MI->isInlineAsm())
     return false;
 
-  // Don't delete frame allocation labels.
-  if (MI->getOpcode() == TargetOpcode::LOCAL_ESCAPE)
-    return false;
-
   // Don't delete instructions with side effects.
   bool SawStore = false;
   if (!MI->isSafeToMove(nullptr, SawStore) && !MI->isPHI())
     return false;
 
   // Examine each operand.
   for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
     const MachineOperand &MO = MI->getOperand(i);