Allows backtracking to clear registers instead of pushing and popping
them to restore state.
Redo of 1135 with bug fixed.
git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1156 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Reduces number of pushes when flushing a trace. Some are converted to clears
in the undo-code instead, and some just ignored if they have no value worth restoring.
git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1136 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
