[WebAssembly][NFC] Remove repetition of Defs = [ARGUMENTS]

Summary:
By moving that line into the `I` multiclass.

Reviewers: aheejin

Subscribers: dschuff, sbc100, jgravelle-google, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D53093

llvm-svn: 344180