nfp: bpf: split up the skip flag

We fail program loading if jump lands on a skipped instruction.
This is for historical reasons, it used to be that we only skipped
instructions optimized out based on prior context, and therefore
the optimization would be buggy if we jumped directly to such
instruction (because the context would be skipped by the jump).

There are cases where instructions can be skipped without any
context, for example there is no point in generating code for:

	 r0 |= 0

We will also soon support dropping dead code, so make the skip
logic differentiate between "optimized with preceding context"
vs other skip types.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>