IR: Rename 'operator ==()' to 'isKeyOf()', NFC

`isKeyOf()` is a clearer name than overloading `operator==()`.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228242 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed