Set bailout reason to kCodeGenerationFailed only when it is not set yet

BUG=None
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/22404007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16134 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Allow HPhis to have an invalid merge index.

All phis that do not represent local variables or values on the operand
stack are not allowed to carry a merge index, as the replay of the
HEnvironment during LChunkBuilder time might get out of sync due to
colliding indexes.

R=danno@chromium.org
BUG=v8:2815

Review URL: https://codereview.chromium.org/22494003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16135 ce2b1a6d-e550-0410-aec6-3dcde31c8c00