[TwoAddressInstruction] Update LiveIntervals after rewriting INSERT_SUBREG to COPY

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Summary

Also add subranges to an existing live interval when introducing a new
subreg def.

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This is a prerequisite for D112556 and for the goal of enabling -early-live-intervals by default.

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Ping!

llvm/lib/CodeGen/TwoAddressInstructionPass.cpp

for (MachineBasicBlock::iterator mi = MBB->begin(), me = MBB->end();
         unsigned SubIdx = mi->getOperand(3).getImm();
         mi->RemoveOperand(3);
         assert(mi->getOperand(0).getSubReg() == 0 && "Unexpected subreg idx");
         mi->getOperand(0).setSubReg(SubIdx);
         mi->getOperand(0).setIsUndef(mi->getOperand(1).isUndef());
         mi->RemoveOperand(1);
         mi->setDesc(TII->get(TargetOpcode::COPY));
         LLVM_DEBUG(dbgs() << "\t\tconvert to:\t" << *mi);
+
+        // Update LiveIntervals.
+        if (LIS) {
+          Register Reg = mi->getOperand(0).getReg();
+          LiveInterval &LI = LIS->getInterval(Reg);
+          if (LI.hasSubRanges()) {
+            // The COPY no longer defines subregs of %reg except for
+            // %reg.subidx.
+            LaneBitmask LaneMask =
+                TRI->getSubRegIndexLaneMask(mi->getOperand(0).getSubReg());
+            SlotIndex Idx = LIS->getInstructionIndex(*mi);
+            for (auto &S : LI.subranges()) {
+              if ((S.LaneMask & LaneMask).none()) {
+                LiveRange::iterator UseSeg = S.FindSegmentContaining(Idx);
+                LiveRange::iterator DefSeg = std::next(UseSeg);
+                S.MergeValueNumberInto(DefSeg->valno, UseSeg->valno);
+              }
+            }
+
+            // The COPY no longer has a use of %reg.
+            LIS->shrinkToUses(&LI);
+          } else {
+            // The live interval for Reg did not have subranges but now it needs
+            // them because we have introduced a subreg def. Recompute it.
+            LIS->removeInterval(Reg);
+            LIS->createAndComputeVirtRegInterval(Reg);
+          }
+        }
       }
 
       // Clear TiedOperands here instead of at the top of the loop
       // since most instructions do not have tied operands.
       TiedOperands.clear();
       removeClobberedSrcRegMap(&*mi);
       mi = nmi;
     }

llvm/test/CodeGen/AMDGPU/undefined-subreg-liverange.ll

 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
 ; RUN: llc -march=amdgcn -verify-machineinstrs < %s | FileCheck %s
+; RUN: llc -march=amdgcn -verify-machineinstrs -early-live-intervals < %s | FileCheck %s
 
 ; We may have subregister live ranges that are undefined on some paths. The
 ; verifier should not complain about this.
 
 define amdgpu_kernel void @func() #0 {
 ; CHECK-LABEL: func:
 ; CHECK:       ; %bb.0: ; %B0
 ; CHECK-NEXT:    s_mov_b32 s0, 0