IRVerifier: Correctly check attribute types

Add missing parenthesis such that all and not only the very first attribute
is checked.

Testing this piece of code is not possible with an LLVM-IR test file, as the
LLVM-IR parser has a similar check such that the wrong IR does not even arrive
at the verifier.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185408 91177308-0d34-0410-b5e6-96231b3b80d8