Internal coding: Assert that user#() don't overlap.

Any use of a user() must now be declared, generally in the Visitor class with
    AstUser#InUse   arbitrary_object
This lets the code track if there's another request for the same user(),
preventing nasty hard to debug cases where they overlap.  This will also
call user#ClearTree(), so a bunch of those were removed, though many
extranious ones still remain.

src/V3LifePost.cpp

@@ -98,6 +98,9 @@ private:
     //  AstVarScope::user()	-> Sequence # of first virtex setting this var.
     //  AstVarScope::user2()	-> Sequence # of last consumption of this var
     //  AstVarScope::user4()	-> AstVarScope*: Passed to LifePostElim to substitute this var
+    AstUserInUse	m_inuse1;
+    AstUser2InUse	m_inuse2;
+    AstUser4InUse	m_inuse4;
 
     // STATE
     uint32_t		m_sequence;	// Sequence number of assignments/varrefs
@@ -107,7 +110,6 @@ private:
     virtual void visit(AstTopScope* nodep, AstNUser*) {
 	AstNode::userClearTree();	// userp() used on entire tree
 	AstNode::user2ClearTree();	// userp() used on entire tree
-	AstNode::user3ClearTree();	// userp() used on entire tree
 	AstNode::user4ClearTree();	// userp() used on entire tree
 	m_sequence = 0;
 	nodep->iterateChildren(*this);