[WebAssembly] Change SIMD lane indices to vec_i8imm_op

Summary: To explicitly opt out of LEB encoding for these immediates.

Reviewers: aheejin, dschuff

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

Differential Revision: https://reviews.llvm.org/D51766

llvm-svn: 341707