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/V3Options.cpp

@@ -1497,6 +1497,7 @@ void V3Options::parseOptsList(FileLine* fl, const string& optdir, int argc,
         m_fIcoChangeDetect.setTrueOrFalse(flag);
     });
     DECL_OPTION("-finline", FOnOff, &m_fInline);
+    DECL_OPTION("-finline-cfuncs", FOnOff, &m_fInlineCFuncs);
     DECL_OPTION("-finline-funcs", FOnOff, &m_fInlineFuncs);
     DECL_OPTION("-finline-funcs-eager", FOnOff, &m_fInlineFuncsEager);
     DECL_OPTION("-flife", FOnOff, &m_fLife);
@@ -2371,6 +2372,7 @@ void V3Options::optimize(int level) {
     m_fExpand = flag;
     m_fGate = flag;
     m_fInline = flag;
+    m_fInlineCFuncs = flag;
     m_fLife = flag;
     m_fLifePost = flag;
     m_fLocalize = flag;