MLBEDSW-6915: MLCE - Missing operators in Debug DB

- Adds missing operators and type conversion recording to DebugDB

Change-Id: If76b0b430bbe73ae1469024c3160ecf0eea26abe
Signed-off-by: wilisa01 <william.isaksson@arm.com>
diff --git a/ethosu/vela/compiler_driver.py b/ethosu/vela/compiler_driver.py
index 014d12a..439b954 100644
--- a/ethosu/vela/compiler_driver.py
+++ b/ethosu/vela/compiler_driver.py
@@ -134,7 +134,7 @@
 
 
 def _record_operator(op, arch):
-    if op.type != Op.Const:
+    if op.type not in (Op.Const, Op.Placeholder):
         DebugDatabase.add_source(op)