Skip unreachable blocks when looking for next emitted block.

Goto does not emit a jump if the target is the next emitted block.
However, if there is an unreachable block between Goto and the jump
target, we still emit a jump even though the unreachable block is
not actually emitted. That jump is unnecessary.

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/174883002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19739 ce2b1a6d-e550-0410-aec6-3dcde31c8c00