Linux Sandbox: fix BPF compiler bug

      The code responsible for detecting similar blocks and merging
them didn't check for the next blocks if the last instruction was
not a JMP or a RET.

The patch to fix this bug (in codegen.cc) is based on a patch by
jld@panix.com, attached to the bug report.

Additional unittests are from jln@chromium.org

BUG=351103

Review URL: https://codereview.chromium.org/215173002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@260157 0039d316-1c4b-4281-b951-d872f2087c98