[LiveDebugValues] Disable spill tracking on AMDGPU

The spill tracking in MLocTracker has quite poor performance on AMDGPU
because of the large number of subregister indices. Futhermore, range
extension doesn't even work on spills since AMDGPU currently doesn't
implement is{LoadFrom,StoreTo}StackSlotPostFE. Disable it for now until
we can come up with a better way of handling this.

Fixes SWDEV-511880

Author: epilk

llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp

@@ -156,6 +156,15 @@ static cl::opt<unsigned>
                          cl::desc("livedebugvalues-stack-ws-limit"),
                          cl::init(250));
 
+// Limit for the maximum number of stack slot indexes. On targets where this is
+// exceeded, this effectivly disables tracking debug locations across spills.
+// The spill tracking in MLocTracker performs quite poorly in terms of memory
+// and time on targets with a more complicated register file (FIXME).
+static cl::opt<unsigned>
+    StackSlotIdxesLimit("livedebugvalues-max-stack-slot-idxes", cl::Hidden,
+                        cl::desc("livedebugvalues-max-stack-slot-idxes"),
+                        cl::init(128));
+
 DbgOpID DbgOpID::UndefID = DbgOpID(0xffffffff);
 
 /// Tracker for converting machine value locations and variable values into
@@ -1122,6 +1131,10 @@ void MLocTracker::writeRegMask(const MachineOperand *MO, unsigned CurBB,
 }
 
 std::optional<SpillLocationNo> MLocTracker::getOrTrackSpillLoc(SpillLoc L) {
+  // Disable spill tracking on targets with a large number of slot idxes.
+  if (NumSlotIdxes >= StackSlotIdxesLimit)
+    return std::nullopt;
+
   SpillLocationNo SpillID(SpillLocs.idFor(L));
 
   if (SpillID.id() == 0) {