[X86] Move target-generic code into CodeGen [NFC]

[void]

Summary

This code is the same for all platforms.

llvm/lib/CodeGen/PrologEpilogInserter.cpp

for (MCRegister Reg : AllocatableSet.set_bits()) {
 
     // Want only registers used for arguments.
     if (OnlyArg && !TRI.isArgumentRegister(MF, Reg))
       continue;
 
     RegsToZero.set(Reg);
   }
 
-  // Remove registers that are live when leaving the function.
+  // Don't clear registers that are live when leaving the function.
   for (const MachineBasicBlock &MBB : MF)
     for (const MachineInstr &MI : MBB.terminators()) {
       if (!MI.isReturn())
         continue;
 
       for (const auto &MO : MI.operands()) {
         if (!MO.isReg())
           continue;
 
         for (MCPhysReg SReg : TRI.sub_and_superregs_inclusive(MO.getReg()))
           RegsToZero.reset(SReg);
       }
     }
 
+  // Don't need to clear registers that are used/clobbered by terminating
+  // instructions.
+  for (const MachineBasicBlock &MBB : MF) {
+    if (!MBB.isReturnBlock())
+      continue;
+
+    MachineBasicBlock::const_iterator MBBI = MBB.getFirstTerminator();
+    for (MachineBasicBlock::const_iterator I = MBBI, E = MBB.end(); I != E;
+         ++I) {
+      for (const MachineOperand &MO : I->operands()) {
+        if (!MO.isReg())
+          continue;
+
+        for (const MCPhysReg &Reg :
+             TRI.sub_and_superregs_inclusive(MO.getReg()))
+          RegsToZero.reset(Reg);
+      }
+    }
+  }
+
+  // Don't clear registers that are reset before exiting.
+  for (const CalleeSavedInfo &CSI : MF.getFrameInfo().getCalleeSavedInfo())
+    for (MCRegister Reg : TRI.sub_and_superregs_inclusive(CSI.getReg()))
+      RegsToZero.reset(Reg);
+
   const TargetFrameLowering &TFI = *MF.getSubtarget().getFrameLowering();
   for (MachineBasicBlock &MBB : MF)
     if (MBB.isReturnBlock())
       TFI.emitZeroCallUsedRegs(RegsToZero, MBB);
 }
 
 /// replaceFrameIndices - Replace all MO_FrameIndex operands with physical
 /// register references and actual offsets.

llvm/lib/Target/X86/X86FrameLowering.cpp

for (const CalleeSavedInfo &I : CSI) {
     }
   }
 }
 
 void X86FrameLowering::emitZeroCallUsedRegs(BitVector RegsToZero,
                                             MachineBasicBlock &MBB) const {
   const MachineFunction &MF = *MBB.getParent();
 
-  // Don't clear registers that will just be reset before exiting.
-  for (const CalleeSavedInfo &CSI : MF.getFrameInfo().getCalleeSavedInfo())
-    for (MCRegister Reg : TRI->sub_and_superregs_inclusive(CSI.getReg()))
-      RegsToZero.reset(Reg);
-
   // Insertion point.
   MachineBasicBlock::iterator MBBI = MBB.getFirstTerminator();
 
-  // We don't need to zero out registers that are clobbered by "pop"
-  // instructions.
-  for (MachineBasicBlock::iterator I = MBBI, E = MBB.end(); I != E; ++I)
-    for (const MachineOperand &MO : I->operands()) {
-      if (!MO.isReg())
-        continue;
-
-      for (const MCPhysReg &Reg : TRI->sub_and_superregs_inclusive(MO.getReg()))
-        RegsToZero.reset(Reg);
-    }
-
   // Fake a debug loc.
   DebugLoc DL;
   if (MBBI != MBB.end())
     DL = MBBI->getDebugLoc();
 
   // Zero out FP stack if referenced. Do this outside of the loop below so that
   // it's done only once.
   const X86Subtarget &ST = MF.getSubtarget<X86Subtarget>();