Try to improve FIXME

Author: s-barannikov

llvm/lib/CodeGen/ScheduleDAGInstrs.cpp

@@ -275,9 +275,12 @@ void ScheduleDAGInstrs::addPhysRegDataDeps(SUnit *SU, unsigned OperIdx) {
       bool ImplicitPseudoUse = false;
       SDep Dep;
       if (UseOpIdx < 0) {
-        // FIXME: UseOpIdx can be passed to computeOperandLatency, which can
-        //   pass it to findUseIdx, which treats it as unsigned. If this is
-        //   the expected behavior, it should be commented.
+        // FIXME: UseOpIdx passed to computeOperandLatency below should be
+        //   non-negative. Currently a negative value is passed if UseOpIdx < 0
+        //   and ImplicitPseudoDef is false. This could be fixed by setting
+        //   ImplicitPseudoUse to true here (which is probably the right thing
+        //   to do), but this crashes Hexagon backend and causes many test
+        //   changes that need investigation.
         Dep = SDep(SU, SDep::Artificial);
       } else {
         // Set the hasPhysRegDefs only for physreg defs that have a use within