Teach globalopt to not nuke all stores to globals. Keep them around of they

Teach globalopt to not nuke all stores to globals. Keep them around of they
might be deliberate "one time" leaks, so that leak checkers can find them.
This is a reapply of r160602 with the fix that this time I'm committing the
code I thought I was committing last time; the I->eraseFromParent() goes
*after* the break out of the loop.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@160664 91177308-0d34-0410-b5e6-96231b3b80d8