Cover dist dedup arms and drop unreachable chain guard

src/V3Randomize.cpp

@@ -4566,7 +4566,6 @@ class RandomizeVisitor final : public VNVisitor {
     // frozen variable requires only membership, not a freshly drawn bucket.
     AstNode* gateFrozenDist(AstNode* chainp, AstDist* distp,
                             const std::vector<DistBucket>& buckets, AstVar* randModeVarp) {
-        if (!chainp) return chainp;
         FileLine* const fl = distp->fileline();
         AstNodeExpr* gatep = nullptr;
         std::unordered_set<const AstVar*> gatedVars;

test_regress/t/t_constraint_dist_randmode_frozen.v

@@ -82,6 +82,19 @@ class MultiVar;
   constraint c {(x + y) dist {10 := 9, 1000 := 1};}
 endclass
 
+class DupVar;
+  rand uint x;
+  constraint c {(x + x) dist {10 := 9, 20 := 1};}
+endclass
+
+class DupMember;
+  rand Sub s;
+  constraint c {(s.x + s.x) dist {10 := 9, 20 := 1};}
+  function new();
+    s = new;
+  endfunction
+endclass
+
 class RtWeight;
   rand uint x;
   uint w;
@@ -259,6 +272,48 @@ module t;
       `checkd(m.y, 500);
     end
 
+    // Same var twice in the dist expression, frozen: membership of 2*x decides.
+    begin
+      DupVar d;
+      d = new;
+      ok = d.randomize();
+      `checkd(ok, 1);
+      d.x.rand_mode(0);
+      d.x = 5;
+      for (int i = 0; i < 8; ++i) begin
+        ok = d.randomize();
+        `checkd(ok, 1);
+        `checkd(d.x, 5);
+      end
+      d.x = 7;
+      for (int i = 0; i < 8; ++i) begin
+        ok = d.randomize();
+        `checkd(ok, 0);
+        `checkd(d.x, 7);
+      end
+    end
+
+    // Same sub-object member twice in the dist expression, frozen.
+    begin
+      DupMember dm;
+      dm = new;
+      ok = dm.randomize();
+      `checkd(ok, 1);
+      dm.s.x.rand_mode(0);
+      dm.s.x = 5;
+      for (int i = 0; i < 8; ++i) begin
+        ok = dm.randomize();
+        `checkd(ok, 1);
+        `checkd(dm.s.x, 5);
+      end
+      dm.s.x = 7;
+      for (int i = 0; i < 8; ++i) begin
+        ok = dm.randomize();
+        `checkd(ok, 0);
+        `checkd(dm.s.x, 7);
+      end
+    end
+
     // Runtime weight of zero excludes the bucket's values (18.5.3): frozen
     // there must fail; nonzero weight on the same frozen value succeeds.
     r = new;