[llvm] [AMDGPU] Use some merging/unmerging helpers in SILoadStoreOptimizer (PR #90866)

[Matt Arsenault via llvm-commits]

================
@@ -1191,6 +1199,57 @@ SILoadStoreOptimizer::checkAndPrepareMerge(CombineInfo &CI,
   return Where;
 }
 
+// Copy the merged load result from DestReg to the original dest regs of CI and
+// Paired.
+void SILoadStoreOptimizer::copyToDestRegs(
+    CombineInfo &CI, CombineInfo &Paired,
+    MachineBasicBlock::iterator InsertBefore, int OpName,
+    Register DestReg) const {
+  MachineBasicBlock *MBB = CI.I->getParent();
+  DebugLoc DL = CI.I->getDebugLoc();
+
+  auto [SubRegIdx0, SubRegIdx1] = getSubRegIdxs(CI, Paired);
+
+  // Copy to the old destination registers.
+  const MCInstrDesc &CopyDesc = TII->get(TargetOpcode::COPY);
+  const auto *Dest0 = TII->getNamedOperand(*CI.I, OpName);
+  const auto *Dest1 = TII->getNamedOperand(*Paired.I, OpName);
+
+  BuildMI(*MBB, InsertBefore, DL, CopyDesc)
+      .add(*Dest0) // Copy to same destination including flags and sub reg.
+      .addReg(DestReg, 0, SubRegIdx0);
+  BuildMI(*MBB, InsertBefore, DL, CopyDesc)
+      .add(*Dest1)
+      .addReg(DestReg, RegState::Kill, SubRegIdx1);
+}
+
+// Return a register for the source of the merged store after copying the
+// originalsource regs of CI and Paired into it.
----------------
arsenm wrote:

```suggestion
// original source regs of CI and Paired into it.
```

https://github.com/llvm/llvm-project/pull/90866