Only look at the --ignition-generators flag when determining whether to use
Ignition (in compiler.cc).  In generator runtime functions, instead of looking
at the flag, determine the generator kind based on whether the generator has a
bytecode array.  This allows compiling some generator function using
full-codegen and others using Ignition, e.g when using --ignition-filter.

R=mstarzinger@chromium.org
BUG=chromium:618657,v8:4907

Review-Url: https://codereview.chromium.org/2052873002
Cr-Commit-Position: refs/heads/master@{#36864}