[globalisel][tablegen] Fix future undefined behaviour in r316463.

I missed a dereference of `Matched` that preceeded the new check. Thanks to
Justin Bogner for spotting it.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316480 91177308-0d34-0410-b5e6-96231b3b80d8