Improve the tree builder in IR optimisation. Allow load expressions to be

Improve the tree builder in IR optimisation. Allow load expressions to be
moved past Put/I statements and dirty helpers, when it is safe to do so.
It is safe, when the statement does not require exact memory exceptions.
New functions stmt_modifies_guest_state and dirty_helper_puts have been
added to determine the side effect on the guest state.
This optimisation enables the use of  memory-to-memory insns on
architectures that have those.

git-svn-id: svn://svn.valgrind.org/vex/trunk@2645