Optimize instance port connections by aliasing rather than by assignment (#8427)

Use AstAlias instead of AstAssignW in V3Inst port connection lowering,
the same way as V3Inline would do if the instance is inlined. This
closes some of the gap between -finline and -fno-inline behaviour, and
is also required for moving V3Inline post scope, where it could not
create aliases anymore (hence required for future patch to maintain
current behaviour).

This is also a partial fix to #4698 (only when the connected expression
is a plain VarRef)

src/V3FsmDetect.cpp

@@ -599,8 +599,9 @@ class FsmDetectVisitor final : public VNVisitor {
             if (!fsmRegisterWrapperDesc(cellp)) return;
             UASSERT_OBJ(lhsVscp->varp()->isInput(), nodep,
                         "Child-side port alias lhs should be an input");
-            UASSERT_OBJ(rhsVscp->scopep() == m_scopep, nodep,
-                        "Child input port alias should connect from the parent scope");
+            // Note the connected variable can live in a scope further up, as V3Inst
+            // aliases a port connection, merging the port variable of an instance into
+            // whatever it is connected to
             m_cellPortAliases[cellp][lhsVscp->varp()->name()] = rhsVscp;
             m_cellPortChildAliases[cellp][lhsVscp->varp()->name()] = lhsVscp;
             addWrapperCell(m_scopep, cellp);
@@ -608,8 +609,6 @@ class FsmDetectVisitor final : public VNVisitor {
             if (!fsmRegisterWrapperDesc(cellp)) return;
             UASSERT_OBJ(rhsVscp->varp()->isWritable(), nodep,
                         "Child-side port alias rhs should be writable");
-            UASSERT_OBJ(lhsVscp->scopep() == m_scopep, nodep,
-                        "Child output port alias should connect into the parent scope");
             m_cellPortAliases[cellp][rhsVscp->varp()->name()] = lhsVscp;
             m_cellPortChildAliases[cellp][rhsVscp->varp()->name()] = rhsVscp;
             addWrapperCell(m_scopep, cellp);