Make AAMDNodes ctor and operator bool (!!!) explicit, mop up bugs and weirdness exposed by it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219068 91177308-0d34-0410-b5e6-96231b3b80d8
[SystemZ] Make operator bool explicit. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219069 91177308-0d34-0410-b5e6-96231b3b80d8