Make adding nocapture a bit stronger. FreeInst is nocapture. Also,

Make adding nocapture a bit stronger. FreeInst is nocapture. Also,
functions that don't write can't leak a pointer except through
the return value, so a void readonly function is implicitly nocapture.

Test these, and add a test that verifies that f1 calling f2 with an
otherwise dead pointer gets both of them marked nocapture.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@61552 91177308-0d34-0410-b5e6-96231b3b80d8