COFF: De-virtualize and inline garbage collector functions.

isRoot, isLive and markLive functions are called very frequently.
Previously, they were virtual functions. This patch make them
non-virtual.

Also this patch checks chunk liveness before calling its mark().
Previously, we did that at beginning of markLive(), so the virtual
function would return immediately if it's live. That was inefficient.

llvm-svn: 239458