Merge branch 'dead-code-elimination'

Jakub Kicinski says:

====================
This set adds support for complete removal of dead code.

Patch 3 contains all the code removal logic, patches 2 and 4
additionally optimize branches around and to dead code.

Patches 6 and 7 allow offload JITs to take advantage of the
optimization.  After a few small clean ups (8, 9, 10) nfp
support is added (11, 12).

Removing code directly in the verifier makes it easy to adjust
the relevant metadata (line info, subprogram info).  JITs for
code store constrained architectures would have hard time
performing such adjustments at JIT level.  Removing subprograms
or line info is very hard once BPF core finished the verification.
For user space to perform dead code removal it would have to perform
the execution simulation/analysis similar to what the verifier does.

v3:
 - fix uninitilized var warning in GCC 6 (buildbot).
v4:
 - simplify the linfo-keeping logic (Yonghong).  Instead of
   trying to figure out that we are removing first instruction
   of a subprogram, just always keep last dead line info, if
   first live instruction doesn't have one.
v5:
 - improve comments (Martin Lau).
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>