Make DAE not wipe out attributes on calls, and not drop

Make DAE not wipe out attributes on calls, and not drop
return attributes on the floor.  In the case of a call
to a varargs function where the varargs arguments are
being removed, any call attributes on those arguments
need to be dropped.  I didn't do this because I plan to
make it illegal to have such attributes (see next patch).
With this change, compiling the gcc filter2 eh test at -O0
and then running opt -std-compile-opts on it results in
a correctly working program (compiling at -O1 or higher
results in the test failing due to a problem with how we
output eh info into the IR).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@45285 91177308-0d34-0410-b5e6-96231b3b80d8