[WebAssembly] Made disassembler only use stack instructions.

Summary:
Now uses the StackBased bit from the tablegen defs to identify
stack instructions (and ignore register based or non-wasm instructions).

Also changed how we store operands, since we now have up to 16 of them
per instruction. To not cause static data bloat, these are compressed
into a tiny table.

+ a few other cleanups.

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

Reviewers: dschuff, jgravelle-google, sunfish, tlively

Subscribers: sbc100, aheejin, llvm-commits

Differential Revision: https://reviews.llvm.org/D51320

llvm-svn: 341081