[WebAssembly][NFC] Standardize SIMD multiclass format

Summary:
This CL changes the ExtractLane ISEL multiclass to more closely mirror
the structure of the splat and replace_lane multiclasses.

Reviewers: aheejin, dschuff

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

Differential Revision: https://reviews.llvm.org/D50794

llvm-svn: 339801