Improve handling of dirty helper calls when building trees in ado_treebuild_BB.

Improve handling of dirty helper calls when building trees in ado_treebuild_BB.
This function took an overly conservative approach and always assumed
that calling a dirty helper would modify both guest state and memory. This
patch introduces two new functions dirty_helper_stores and dirty_helper_puts,
to determine the actual side effets of a helper call. Using these functions
increases precision and allows the tree builder to move a GET past a dirty
helper call.

git-svn-id: svn://svn.valgrind.org/vex/trunk@2625