[objc-arc] Fix assertion in EraseInstruction so that noop on null calls when passed null do not trigger the assert.

The specific case of interest is when objc_retainBlock is passed null.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185885 91177308-0d34-0410-b5e6-96231b3b80d8