Added NO_COMB annotation.

vlog_to_model in some cases (e.g. DFF with enable signal) incorrectly
detects signals as combinational. This annotation explicitly specifies
input as non-combinational.

Signed-off-by: Maciej Kurc <mkurc@antmicro.com>

v2x/vlog_to_model.py

diff --git a/v2x/vlog_to_model.py b/v2x/vlog_to_model.py
index 0a0a7dc..42b6ef2 100755
--- a/v2x/vlog_to_model.py
+++ b/v2x/vlog_to_model.py
@@ -172,6 +172,7 @@
             clocks = run.list_clocks(infiles, top)
 
             for name, width, bits, iodir in ports:
+                nocomb = tmod.net_attr(name, "NO_COMB")
                 attrs = dict(name=name)
                 sinks = run.get_combinational_sinks(infiles, top, name)
 
@@ -186,7 +187,7 @@
                     attrs["is_clock"] = "1"
                 else:
                     clks = list()
-                    if len(sinks) > 0 and iodir == "input":
+                    if len(sinks) > 0 and iodir == "input" and nocomb is None:
                         attrs["combinational_sink_ports"] = " ".join(sinks)
                     for clk in clocks:
                         if is_clock_assoc(infiles, top, clk, name, iodir):