[WebAssembly][NFC] Move SIMD encoding tests to dedicated file

Summary:
This change makes the tests more focused and avoids problematic
interactions between the testing modes and instruction encoding. This
change also allows the other tests to use less verbose output and
stricter checks.

Reviewers: aheejin, dschuff, aardappel

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

Differential Revision: https://reviews.llvm.org/D52007

llvm-svn: 342287