[XRay][tools] Remove some assertions in llvm-xray graph

[dberris]

Summary

Assertions assuming that function calls may not have zero durations do
not seem to hold in the wild. There are valid cases where the conversion
of the tsc counters end up becoming zero-length durations. These
assertions don't really hold and the algorithms don't need those to be
true for them to work.

test/tools/llvm-xray/X86/graph-zero-latency-calls.yaml

+#RUN: llvm-xray graph %s -o - -m %S/Inputs/simple-instrmap.yaml | FileCheck %s
+
+---
+header:
+  version: 1
+  type: 0
+  constant-tsc: true
+  nonstop-tsc: true
+  cycle-frequency: 2601000000
+records:
+  - { type: 0, func-id: 1, cpu: 1, thread: 111, kind: function-enter, tsc: 10001 }
+  - { type: 0, func-id: 2, cpu: 1, thread: 111, kind: function-enter, tsc: 10002 }
+  - { type: 0, func-id: 2, cpu: 1, thread: 111, kind: function-exit, tsc: 10002 }
+  - { type: 0, func-id: 1, cpu: 1, thread: 111, kind: function-exit, tsc: 10100 }
+...
+
+#CHECK:     digraph xray {
+#CHECK-DAG:   F0 -> F1 [{{.*}}];
+#CHECK-DAG:   F1 -> F2 [{{.*}}];
+#CHECK-DAG: }

tools/llvm-xray/xray-graph.cc

Error GraphRenderer::accountRecord(const XRayRecord &Record) {
   }
   }
 
   return Error::success();
 }
 
 template <typename U>
 void GraphRenderer::getStats(U begin, U end, GraphRenderer::TimeStat &S) {
-  assert(begin != end);
+  if (begin == end) return;
   std::ptrdiff_t MedianOff = S.Count / 2;
   std::nth_element(begin, begin + MedianOff, end);
   S.Median = *(begin + MedianOff);
   std::ptrdiff_t Pct90Off = (S.Count * 9) / 10;
   std::nth_element(begin, begin + Pct90Off, end);
   S.Pct90 = *(begin + Pct90Off);
   std::ptrdiff_t Pct99Off = (S.Count * 99) / 100;
   std::nth_element(begin, begin + Pct99Off, end);
void GraphRenderer::updateMaxStats(const GraphRenderer::TimeStat &S,
   M.Sum = std::max(M.Sum, S.Sum);
 }
 
 void GraphRenderer::calculateEdgeStatistics() {
   assert(!G.edges().empty());
   for (auto &E : G.edges()) {
     auto &A = E.second;
     assert(!A.Timings.empty());
-    assert((A.Timings[0] > 0));
     getStats(A.Timings.begin(), A.Timings.end(), A.S);
-    assert(A.S.Sum > 0);
     updateMaxStats(A.S, G.GraphEdgeMax);
   }
 }
 
 void GraphRenderer::calculateVertexStatistics() {
   std::vector<uint64_t> TempTimings;
   for (auto &V : G.vertices()) {
-    assert((V.first == 0 || G[V.first].S.Sum != 0) &&
-           "Every non-root vertex should have at least one call");
     if (V.first != 0) {
       for (auto &E : G.inEdges(V.first)) {
         auto &A = E.second;
         TempTimings.insert(TempTimings.end(), A.Timings.begin(),
                            A.Timings.end());
       }
-      assert(!TempTimings.empty() && TempTimings[0] > 0);
       getStats(TempTimings.begin(), TempTimings.end(), G[V.first].S);
       updateMaxStats(G[V.first].S, G.GraphVertexMax);
       TempTimings.clear();
     }
   }
 }
 
 // A Helper function for normalizeStatistics which normalises a single