Don't enqueue onto a disabled dep_graph.
This commit guards all calls to `DepGraphThreadData::enqueue` with a
check to make sure it is enabled. This avoids some useless allocation
and vector manipulations when it is disabled (i.e. when incremental
compilation is off) which improves speed by 1--2% on most of the
rustc-benchmarks.
This commit has an observable functional change: when the dep_graph is
disabled its `shadow_graph` will no longer receive messages. This should
be ok because these message are only used when debug assertions are
enabled.
r? @nikomatsakis
eb38d426c4