Experimental: statically distinguish between named basic block entries...

Experimental: statically distinguish between named basic block entries
that are definitely reached by only forward CFG edges and those that
may be reached by a backward edge.  Assert that we do not try to jump
backward to forward-only blocks.  Otherwise, the two types of blocks
are not handled differently yet.

There are only a handful of backward edges:
 * one for each of the loop types (while, for, and do...while),
 * the for...in statement,
 * a loop in GenerateFastCharCodeAt,
 * the fall-through from a default case in a switch when it is not the
   last case,
 * (possibly) the return sequence
 * the exit labels from deferred code

The last two are not true backward edges (there is no loop).

Committed: http://code.google.com/p/v8/source/detail?r=1121

[Kevin Millikin (Chromium), 2009-01-20 12:04:49]

The default argument actually seems more explicit, because the backward edges
stand out more when reading the code.

[William Hesse, 2009-01-21 07:59:12]

LGTM