Add --instr-count-dpi option, change default to 200

This replaces the former static AstNode::INSTR_COUNT_DPI, and makes it
user adjustable to fit the design.

Fixes #3068.

src/V3Ast.h

@@ -1524,7 +1524,6 @@ public:
     static constexpr int INSTR_COUNT_STR = 100;  // String ops
     static constexpr int INSTR_COUNT_TIME = INSTR_COUNT_CALL + 5;  // Determine simulation time
     static constexpr int INSTR_COUNT_PLI = 20;  // PLI routines
-    static constexpr int INSTR_COUNT_DPI = 1000;  // DPI import function
 
     // ACCESSORS
     virtual string name() const { return ""; }

src/V3AstNodes.h

@@ -8788,7 +8788,9 @@ public:
     }
     //
     virtual void name(const string& name) override { m_name = name; }
-    virtual int instrCount() const override { return dpiImportPrototype() ? INSTR_COUNT_DPI : 0; }
+    virtual int instrCount() const override {
+        return dpiImportPrototype() ? v3Global.opt.instrCountDpi() : 0;
+    }
     VBoolOrUnknown isConst() const { return m_isConst; }
     void isConst(bool flag) { m_isConst.setTrueOrFalse(flag); }
     void isConst(VBoolOrUnknown flag) { m_isConst = flag; }

src/V3Options.cpp

@@ -1101,6 +1101,10 @@ void V3Options::parseOptsList(FileLine* fl, const string& optdir, int argc, char
     DECL_OPTION("-if-depth", Set, &m_ifDepth);
     DECL_OPTION("-ignc", OnOff, &m_ignc);
     DECL_OPTION("-inline-mult", Set, &m_inlineMult);
+    DECL_OPTION("-instr-count-dpi", CbVal, [this, fl](int val) {
+        m_instrCountDpi = val;
+        if (m_instrCountDpi < 0) fl->v3fatal("--instr-count-dpi must be non-negative: " << val);
+    });
 
     DECL_OPTION("-LDFLAGS", CbVal, callStrSetter(&V3Options::addLdLibs));
     const auto setLang = [this, fl](const char* valp) {

src/V3Options.h

@@ -289,6 +289,7 @@ private:
     int         m_gateStmts = 100;    // main switch: --gate-stmts
     int         m_ifDepth = 0;      // main switch: --if-depth
     int         m_inlineMult = 2000;   // main switch: --inline-mult
+    int         m_instrCountDpi = 200;   // main switch: --instr-count-dpi
     VOptionBool m_makeDepend;  // main switch: -MMD
     int         m_maxNumWidth = 65536;  // main switch: --max-num-width
     int         m_moduleRecursion = 100;  // main switch: --module-recursion-depth
@@ -489,6 +490,7 @@ public:
     int gateStmts() const { return m_gateStmts; }
     int ifDepth() const { return m_ifDepth; }
     int inlineMult() const { return m_inlineMult; }
+    int instrCountDpi() const { return m_instrCountDpi; }
     VOptionBool makeDepend() const { return m_makeDepend; }
     int maxNumWidth() const { return m_maxNumWidth; }
     int moduleRecursionDepth() const { return m_moduleRecursion; }