tblgen: Reuse function that is 2 lines above.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171937 91177308-0d34-0410-b5e6-96231b3b80d8
Inline this into its only caller.

It's clearer and additionally this gets rid of the usage of `DefmID`,
which doesn't really correspond to anything in the language (it was just
used in the name of this parsing function which parsed a `MultiClassID`
and returned that multiclass's record).

This area of the code still needs a lot of work.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171938 91177308-0d34-0410-b5e6-96231b3b80d8