Fix a bug with uninitialized const variables in the optimizing compiler.

Fix a bug with uninitialized const variables in the optimizing compiler.

We have to check for uninitialized uses before phi-elimination. Otherwise we
may miss such a use and result in using the hole value instead. This
causes a NULL-dereference or assertion failure.

BUG=96989
TEST=mjsunit/compiler/regress-96989.js
Committed: http://code.google.com/p/v8/source/detail?r=9337

[Kevin Millikin (Chromium), 2011-09-20 09:03:46]

This LGTM as a bugfix.

But... we have seen a performance regression if we check for arguments before
eliminating redundant phis.

I guess we should consider (1) check for uninitialized const, (2) eliminate
redundant phis, (3) check for arguments.

[fschneider, 2011-09-20 09:42:23]

Good point. I uploaded a new version that checks for arguments-uses in the right
place.

[Kevin Millikin (Chromium), 2011-09-20 10:00:31]

LGTM.