Discard unused nesting levels.

Discard unused nesting levels.

This avoids the line splitter trying nesting levels that will never be
selected. On some pathologically deeply nested code, this can
make a huge (1000x) perf difference. Most code is unaffected code,
but this still make a 10% overall improvement on the original
benchmark that didn't stress the worst case performance of this.

R=pquitslund@google.com

Committed: db6b9bfb6594ea095278d50aed164a096a09346f

[Bob Nystrom, 2015-03-09 23:02:38]

rnystrom@google.com changed reviewers:
+ pquitslund@google.com

[Bob Nystrom, 2015-03-09 23:02:38]

This doesn't fix the performance problem on pathologically nested code, but it
makes a big difference one some examples.

[pquitslund, 2015-03-09 23:51:47]

lgtm.

Thanks!

[Bob Nystrom, 2015-03-10 01:14:10]

Committed patchset #1 (id:1) manually as
db6b9bfb6594ea095278d50aed164a096a09346f (presubmit successful).