[WebAssembly] Simplified selecting asmmatcher stack instructions.

Summary:
By using the existing isCodeGenOnly bit in the tablegen defs, as
suggested by tlively in https://reviews.llvm.org/D51662

Tested: llvm-lit -v `find test -name WebAssembly`

Reviewers: tlively

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

Differential Revision: https://reviews.llvm.org/D52373

llvm-svn: 342772