V3Randomize: Fix dist operator inside ConstraintIf blocks (#7221) (#7224)

The lowerDistConstraints() function was not recursing into ConstraintIf
nodes, causing dist operators inside if-else blocks to remain unlowered
and trigger an internal error when ConstraintExprVisitor encountered them.

Fix by adding recursive handling of ConstraintIf nodes in lowerDistConstraints:
- Check for AstConstraintIf nodes before AstConstraintExpr
- Recursively process thensp() and elsesp() branches
- This ensures all dist operators are lowered regardless of nesting

Test case: t_randomize_dist_conditional.v demonstrates conditional dist:
    constraint c {
      if (randd) {
        x dist { 8'd0 := 1, 8'd255 := 3 };  // 25% / 75%
      } else {
        x dist { 8'd0 := 3, 8'd255 := 1 };  // 75% / 25%
      }
    }

Fixes #7221

src/V3Randomize.cpp

@@ -3739,6 +3739,14 @@ class RandomizeVisitor final : public VNVisitor {
     void lowerDistConstraints(AstTask* taskp, AstNode* constrItemsp) {
         for (AstNode *nextip, *itemp = constrItemsp; itemp; itemp = nextip) {
             nextip = itemp->nextp();
+
+            // Recursively handle ConstraintIf nodes (dist can be inside if/else)
+            if (AstConstraintIf* const cifp = VN_CAST(itemp, ConstraintIf)) {
+                if (cifp->thensp()) lowerDistConstraints(taskp, cifp->thensp());
+                if (cifp->elsesp()) lowerDistConstraints(taskp, cifp->elsesp());
+                continue;
+            }
+
             AstConstraintExpr* const constrExprp = VN_CAST(itemp, ConstraintExpr);
             if (!constrExprp) continue;
             AstDist* const distp = VN_CAST(constrExprp->exprp(), Dist);