rule: never merge across non-expression statements

rule: never merge across non-expression statements

The existing logic can merge across non-expression statements,
if there is only one payload expression.

Example:
ether saddr 00:11:22:33:44:55 counter ether type 8021q

is turned into
counter ether saddr 00:11:22:33:44:55 ether type 8021q

which isn't the same thing.

Fix this up and add test cases for adjacent vlan and ip header
fields.  'Counter' serves as a non-merge fence.

Signed-off-by: Florian Westphal <fw@strlen.de>