compiler: do not emit CONVERT for syscall results

When we encounter type assertion CONVERT is emitted.
This isn't needed for SYSCALL (or opcode) results
because value already has needed type.
Problems can arise when result is converted to invalid type
but `neogointernal` package shouldn't be used directly anyway.

pkg/compiler/codegen.go

@@ -1226,6 +1226,10 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 	// not the assertion type.
 	case *ast.TypeAssertExpr:
 		ast.Walk(c, n.X)
+		if c.isCallExprSyscall(n.X) {
+			return nil
+		}
+
 		goTyp := c.typeOf(n.Type)
 		if canConvert(goTyp.String()) {
 			typ := toNeoType(goTyp)
@@ -1246,6 +1250,20 @@ func (c *codegen) packVarArgs(n *ast.CallExpr, typ *types.Signature) int {
 	return varSize
 }
 
+func (c *codegen) isCallExprSyscall(e ast.Expr) bool {
+	ce, ok := e.(*ast.CallExpr)
+	if !ok {
+		return false
+	}
+	sel, ok := ce.Fun.(*ast.SelectorExpr)
+	if !ok {
+		return false
+	}
+	name, _ := c.getFuncNameFromSelector(sel)
+	f, ok := c.funcs[name]
+	return ok && isSyscall(f)
+}
+
 // processDefers emits code for `defer` statements.
 // TRY-related opcodes handle exception as follows:
 // 1. CATCH block is executed only if exception has occurred.