[WebAssembly] Fix encoding of non-SIMD vector-typed instructions

Previously SIMD_I was the same as a normal instruction except for the
addition of a HasSIM128 predicate. However, rL339186 changed the
encoding of SIMD_I instructions to automatically contain the SIMD
prefix byte. This broke the encoding of non-SIMD vector-typed
instructions, which had instantiated SIMD_I. This CL corrects this
error.

Reviewers: aheejin

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

Differential Revision: https://reviews.llvm.org/D50682

Patch by Thomas Lively (tlively)

llvm-svn: 339710