add notes

vyper/venom/ir_node_to_venom.py

@@ -127,7 +127,11 @@ def ir_node_to_venom(ir: IRnode) -> IRContext:
 
     ctx.chain_basic_blocks()
 
-    # errors in make_ssa whether this is commented or not
+    # float allocas to the front of the function. we could probably move
+    # them to the immediate dominator of the basic block defining the alloca
+    # instead of the entry (which dominates all basic blocks), but this is
+    # done for expedience. without this step, sccp fails, possibly because
+    # dominators are not guaranteed to be traversed first.
     ctx.float_allocas()
 
     return ctx