Optimize generated function inlining (#7811)

Previously V3InlineCFuncs inlined call sites but never deleted the now
dead callees. Also missed a lot of opportunities due to evaluation order.

Rewrite using a graph based algorithm, using only a single traversal of
the netlist. This is clearer, more accurate, and faster at compile time.

Also add a clean -fno-inline-cfuncs disable. Setting the limits to 0
still disables inlining, except of empty functions, which can be inlined
with 0 limits (they are no ops). It will also prune unused functions
without -fno-inline-cfuncs.

Pass now also respects `--output-split`

src/Verilator.cpp

@@ -582,8 +582,14 @@ static void process() {
                 // Must be after all Sel/array index based optimizations
                 V3Reloop::reloopAll(v3Global.rootp());
             }
+        }
 
-            if (v3Global.opt.inlineCFuncs()) {
+        // These are no longer needed, remove references before CFunc inlining
+        v3Global.rootp()->evalp(nullptr);
+        v3Global.rootp()->evalNbap(nullptr);
+
+        if (!v3Global.opt.lintOnly() && !v3Global.opt.serializeOnly()) {
+            if (v3Global.opt.fInlineCFuncs()) {
                 // Inline small CFuncs to reduce function call overhead
                 V3InlineCFuncs::inlineAll(v3Global.rootp());
             }