Attempt some loop opts after write barrier expansion

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/shenandoah/wbloopopts/webrev.00/

This attempts a few more rounds of loop opts after write barrier
expansion.

 When there are 2 write barriers in a row, the evacuation in progress
tests are merged:

if (evac_in_progress) {
  slow_path_1
} else {
  fast_path_1
}
if (evac_in_progress) {
  slow_path_2
} else {
  fast_path_2
}

becomes:

if (evac_in_progress) {
  slow_path_1
  slow_path_2
} else {
  fast_path_1
  fast_path_2
}

Loops are unswitched when they contain an evacuation in progress test
that can be moved out of the loop (i.e. no safepoint =
-UseCountedLoopSafepoints).

for (;;) {
  some_stuff
  if (evac_in_progress) {
    slow_path
  } else {
    fast_path
  }
  more_stuff
}

becomes

if (evac_in_progress) {
  for (;;) {
    some_stuff
    slow_path
    more_stuff
  }
} else {
  for (;;) {
    some_stuff
    fast_path
    more_stuff
  }
}

Roland.