Fix LHS is not a lvalue when using tristate with -Oi.

src/V3Tristate.cpp

@@ -1107,6 +1107,7 @@ class TristateVisitor final : public TristateBaseVisitor {
                 AstVar* enVarp = new AstVar(nodep->fileline(), AstVarType::MODULETEMP,
                                             nodep->name() + "__en" + cvtToStr(m_unique++),
                                             VFlagBitPacked(), enModVarp->width());
+                enModVarp->direction(VDirection::INPUT);
                 UINFO(9, "       newenv " << enVarp << endl);
                 AstPin* enpinp
                     = new AstPin(nodep->fileline(), nodep->pinNum(),

test_regress/t/t_altera_lpm_mult_noinl.pl

@@ -0,0 +1,22 @@
+#!/usr/bin/env perl
+if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); die; }
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2003 by Wilson Snyder. This program is free software; you
+# can redistribute it and/or modify it under the terms of either the GNU
+# Lesser General Public License Version 3 or the Perl Artistic License
+# Version 2.0.
+# SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+scenarios(vlt_all => 1);
+
+top_filename("t/t_altera_lpm.v");
+(my $module = $Self->{name}) =~ s/.*t_altera_//;
+$module =~ s/_noinl//;
+
+compile(
+    verilator_flags2 => ["--top-module ${module}", "-Oi"]
+    );
+
+ok(1);
+1;