[WebAssembly] Use 'I' multiclass template for br_table (NFC)

Summary:
We don't need to explicitly use `NI` anymore because we now don't use
`let` statements within the definitions.

Reviewers: aardappel

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

Differential Revision: https://reviews.llvm.org/D56376

llvm-svn: 350594