extend liveness guarantees of client saved registers

_From [derek.br...@gmail.com](https://code.google.com/u/117968039472581148324/) on February 24, 2009 14:41:19_

this was PR 315355

Xref PR 312055. Currently we document that client saved registers remain
saved only until the next application instruction. For one of the TLS slots
and most of the dcontext slots we can actually make the claim that saved
registers remain valid till the next application cti and we could probably
improve on this (excepting the selfmod case we really don't touch many of
the spill slots). This case is about exposing which slots are persisted
longer to the client so it can make use of them.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=53_