More fiddling in an attempt to make it work properly (so far

More fiddling in an attempt to make it work properly (so far
unsuccessful):

* in clean helper calls, take notice of exclusion-mask (mcx_mask)
  specifying which args to ignore.  Seems to have no effect.

* Add Paul-Mackerras-style expensive-add.  Seems to have no effect.

* When doing a shadow PUT, do not complain if uninitialised data
  is written to guest state areas which are marked in the layout info
  as 'always defined'.  This seems to help reduce the false positive
  rate, because (in guest x86) the CC_AUX field of the flags thunk
  is sometimes given an undefined value (old C flag) when setting
  the thunk after inc/dec.  Since we assume that no compiler would
  examine the C flag after inc/dec unless it knew the C flag before it
  was defined (inc/dec do not change C), this is assumed to be safe.

git-svn-id: svn://svn.valgrind.org/vex/trunk@506