In mc_translate a NULL guard expression is sometimes passed around
functions to indicate a "true" expression. That caused some confusion
and led people to believe believe, that IRDirty::guard could be NULL.
It cannot.
This confusion was indirectly spotted by coverity's checker who figured
out that IRDirty::guard was sometimes unconditionally dereferenced
and sometimes compared against NULL.
Cleaning this up...


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13250 a5019735-40e9-0310-863c-91ae7b9d1cf9