Fix `isBuffer` (#329)

* Add support for "well" direction type (nwell, pwell, etc.), and fix isBuffer (+ other functions) to accommodate wells

* Just fix isBuffer issue

liberty/Liberty.cc

@@ -1226,7 +1226,8 @@ LibertyCell::bufferPorts(// Return values.
       }
       output = port;
     }
-    else if (!dir->isPowerGround()) {
+    else if (!port->isPwrGnd()) {
+        // Invalid direction.
 	input = nullptr;
 	output = nullptr;
 	break;

test/get_is_buffer.ok

@@ -0,0 +1 @@
+buf_inst

test/get_is_buffer.tcl

@@ -0,0 +1,5 @@
+# is_buffer property
+read_liberty ../examples/sky130hd_tt.lib.gz
+read_verilog get_is_buffer.v
+link_design dut
+report_object_full_names [get_cells -filter is_buffer]

test/get_is_buffer.v

@@ -0,0 +1,11 @@
+module dut (
+	input           A,
+	output          Y
+);
+
+	sky130_fd_sc_hd__buf_2 buf_inst (
+		.A(A),
+		.X(Y)
+	);
+
+endmodule

test/regression_vars.tcl

@@ -140,6 +140,7 @@ record_example_tests {
 record_sta_tests {
   disconnect_mcp_pin
   get_filter
+  get_is_buffer
   get_is_memory
   get_lib_pins_of_objects
   get_noargs