v0_8: clear expression look aside for transient thread.

This is a back port of code from the development branch. The problem is
that for named blocks (which create a transient thread) the look aside
was not being cleared for the new thread. This resulted in an invalid
variable reference optimization.

tgt-vvp/vvp_process.c

@@ -759,6 +759,10 @@ static int show_stmt_block_named(ivl_statement_t net, ivl_scope_t scope)
       fprintf(vvp_out, "    %%jmp t_%u;\n", out_id);
       fprintf(vvp_out, "t_%u ;\n", sub_id);
 
+	/* The statement within the fork is in a new thread, so no
+	  expression lookaside is valid. */
+      clear_expression_lookaside();
+
       rc = show_stmt_block(net, subscope);
       fprintf(vvp_out, "    %%end;\n");