[globalisel][cse] Merge debug locations when CSE'ing

[dsanders]

I currently don't have an in-tree test case for this but it should be possible to construct one. Essentially it needs CSE enabled, and needs two instructions that need legalization and emit a common instruction during their legalizations with identical constraints (to avoid the copy) and have conflicting debug locations. I'll try to make an AArch64 test case when I get chance.

llvm/lib/CodeGen/GlobalISel/CSEMIRBuilder.cpp

 //===-- llvm/CodeGen/GlobalISel/CSEMIRBuilder.cpp - MIBuilder--*- C++ -*-==//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 /// \file
 /// This file implements the CSEMIRBuilder class which CSEs as it builds
 /// instructions.
 //===----------------------------------------------------------------------===//
 //
 
 #include "llvm/CodeGen/GlobalISel/CSEMIRBuilder.h"
 #include "llvm/CodeGen/GlobalISel/GISelChangeObserver.h"
+#include "llvm/IR/DebugInfoMetadata.h"
 
 using namespace llvm;
 
 bool CSEMIRBuilder::dominates(MachineBasicBlock::const_iterator A,
                               MachineBasicBlock::const_iterator B) const {
   auto MBBEnd = getMBB().end();
   if (B == MBBEnd)
     return true;
CSEMIRBuilder::generateCopiesIfRequired(ArrayRef<DstOp> DstOps,
                                         MachineInstrBuilder &MIB) {
   assert(checkCopyToDefsPossible(DstOps) &&
          "Impossible return a single MIB with copies to multiple defs");
   if (DstOps.size() == 1) {
     const DstOp &Op = DstOps[0];
     if (Op.getDstOpKind() == DstOp::DstType::Ty_Reg)
       return buildCopy(Op.getReg(), MIB.getReg(0));
   }
+
+  // If we didn't generate a copy then we're re-using an existing node directly
+  // instead of emitting any code. Merge the debug location we wanted to emit
+  // into the instruction we're CSE'ing with. Debug locations arent part of the
+  // profile so we don't need to recompute it.
+  if (getDebugLoc()) {
+    GISelChangeObserver *Observer = getState().Observer;
+    if (Observer)
+      Observer->changingInstr(*MIB);
+    MIB->setDebugLoc(
+        DILocation::getMergedLocation(MIB->getDebugLoc(), getDebugLoc()));
+    if (Observer)
+      Observer->changedInstr(*MIB);
+  }
+
   return MIB;
 }
 
 MachineInstrBuilder CSEMIRBuilder::buildInstr(unsigned Opc,
                                               ArrayRef<DstOp> DstOps,
                                               ArrayRef<SrcOp> SrcOps,
                                               Optional<unsigned> Flag) {
   switch (Opc) {