IntrWriteArgMem implies nocapture, but this wasn't

IntrWriteArgMem implies nocapture, but this wasn't
taken advantage of anywhere.  Change the definition
of IntrWriteArgMem to no longer imply nocapture, and
explicitly add nocapture attributes everywhere (well,
not quite everywhere, because some of these intrinsics
did capture their arguments!).  Also, make clear that
the lack of other side-effects does not exclude doing
volatile loads or stores - the atomic intrinsics do
these, yet they are all marked IntrWriteArgMem (this
change is safe because nothing exploited it).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@64539 91177308-0d34-0410-b5e6-96231b3b80d8