[LiveIntervals] Improve repair after convertToThreeAddress

After TwoAddressInstructionPass calls
TargetInstrInfo::convertToThreeAddress, improve the LiveIntervals repair
to cope with convertToThreeAddress creating more than one new
instruction.

This mostly seems to benefit X86. For example in
test/CodeGen/X86/zext-trunc.ll it converts:

  %4:gr32 = ADD32rr %3:gr32(tied-def 0), %2:gr32, implicit-def dead $eflags

to:

  undef %6.sub_32bit:gr64 = COPY %3:gr32
  undef %7.sub_32bit:gr64_nosp = COPY %2:gr32
  %4:gr32 = LEA64_32r killed %6:gr64, 1, killed %7:gr64_nosp, 0, $noreg

Differential Revision: https://reviews.llvm.org/D110335

llvm/lib/CodeGen/TwoAddressInstructionPass.cpp

@@ -590,16 +590,14 @@ bool TwoAddressInstructionPass::isProfitableToConv3Addr(Register RegA,
 bool TwoAddressInstructionPass::convertInstTo3Addr(
     MachineBasicBlock::iterator &mi, MachineBasicBlock::iterator &nmi,
     Register RegA, Register RegB, unsigned Dist) {
+  MachineInstrSpan MIS(mi, MBB);
   MachineInstr *NewMI = TII->convertToThreeAddress(*mi, LV);
   if (!NewMI)
     return false;
 
   LLVM_DEBUG(dbgs() << "2addr: CONVERTING 2-ADDR: " << *mi);
   LLVM_DEBUG(dbgs() << "2addr:         TO 3-ADDR: " << *NewMI);
 
-  if (LIS)
-    LIS->ReplaceMachineInstrInMaps(*mi, *NewMI);
-
   // If the old instruction is debug value tracked, an update is required.
   if (auto OldInstrNum = mi->peekDebugInstrNum()) {
     // Sanity check.
@@ -618,8 +616,26 @@ bool TwoAddressInstructionPass::convertInstTo3Addr(
                                    std::make_pair(NewInstrNum, NewIdx));
   }
 
+  // If convertToThreeAddress created a single new instruction, assume it has
+  // exactly the same effect on liveness as the old instruction. This is much
+  // more efficient than calling repairIntervalsInRange.
+  bool SingleInst = std::next(MIS.begin(), 2) == MIS.end();
+  if (LIS && SingleInst)
+    LIS->ReplaceMachineInstrInMaps(*mi, *NewMI);
+
+  SmallVector<Register> OrigRegs;
+  if (LIS && !SingleInst) {
+    for (const MachineOperand &MO : mi->operands()) {
+      if (MO.isReg())
+        OrigRegs.push_back(MO.getReg());
+    }
+  }
+
   MBB->erase(mi); // Nuke the old inst.
 
+  if (LIS && !SingleInst)
+    LIS->repairIntervalsInRange(MBB, MIS.begin(), MIS.end(), OrigRegs);
+
   DistanceMap.insert(std::make_pair(NewMI, Dist));
   mi = NewMI;
   nmi = std::next(mi);

llvm/test/CodeGen/X86/zext-trunc.ll

@@ -1,5 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
 ; RUN: llc < %s -mtriple=x86_64-unknown-unknown | FileCheck %s
+; RUN: llc < %s -mtriple=x86_64-unknown-unknown -early-live-intervals -verify-machineinstrs | FileCheck %s
 ; rdar://7570931
 
 define i64 @foo(i64 %a, i64 %b) nounwind {