LiveDebugValues pass incorrectly handles register spill for indirect debug values

[kpdev42]

Summary

Fixes PR: https://bugs.llvm.org/show_bug.cgi?id=51704

[aprantl]

I'm a bit surprised that indirect translates into a DerefAfter and not a DerefBefore but I trust you actually tested that this does right thing :-)

llvm/lib/CodeGen/LiveDebugValues/VarLocBasedImpl.cpp

MachineInstr *BuildDbgValue(MachineFunction &MF) const {
         case MachineLocKind::SpillLocKind: {
           // Spills are indirect DBG_VALUEs, with a base register and offset.
           // Use the original DBG_VALUEs expression to build the spilt location
           // on top of. FIXME: spill locations created before this pass runs
           // are not recognized, and not handled here.
           unsigned Base = Loc.SpillLocation.SpillBase;
           auto *TRI = MF.getSubtarget().getRegisterInfo();
           if (MI.isNonListDebugValue()) {
-            DIExpr =
-                TRI->prependOffsetExpression(DIExpr, DIExpression::ApplyOffset,
-                                             Loc.SpillLocation.SpillOffset);
+            auto Deref = Indirect ? DIExpression::DerefAfter : 0;
+            DIExpr = TRI->prependOffsetExpression(
+                DIExpr, DIExpression::ApplyOffset | Deref,
             Indirect = true;
           } else {
             SmallVector<uint64_t, 4> Ops;
             TRI->getOffsetOpcodes(Loc.SpillLocation.SpillOffset, Ops);
             Ops.push_back(dwarf::DW_OP_deref);
             DIExpr = DIExpression::appendOpsToArg(DIExpr, Ops, I);
           }
           MOs.push_back(MachineOperand::CreateReg(Base, false));

llvm/test/DebugInfo/MIR/X86/live-debug-values-restore.mir

 # is restored as an indirect DBG_VALUE. FIXME: Note that for the intervening
 # period of being a spilt location there is still a missing layer of
 # indirection.
 
 # CHECK-LABEL: name: j
 # CHECK-LABEL: bb.0.entry:
 # CHECK:       DBG_VALUE $rdi, 0, ![[TVAR]], !DIExpression()
 # CHECK-LABEL: bb.1.if.then:
-# CHECK:       DBG_VALUE $rsp, 0, ![[TVAR]], !DIExpression(DW_OP_constu, 8, DW_OP_minus)
+# CHECK:       DBG_VALUE $rsp, 0, ![[TVAR]], !DIExpression(DW_OP_constu, 8, DW_OP_minus, DW_OP_deref)
 # CHECK:       INLINEASM
 # CHECK:       DBG_VALUE ${{[a-zA-Z0-9]+}}, 0, ![[TVAR]], !DIExpression()
 # CHECK-LABEL: bb.2.if.end
 
 name:            j
 tracksRegLiveness: true
 liveins:         
   - { reg: '$rdi', virtual-reg: '' }