[WebAssembly] Added default stack-only instruction mode for MC.

Summary:
Made it convert from register to stack based instructions, and removed the registers.
Fixes to related code that was expecting register based instructions.
Added the correct testing flag to all tests, depending on what the
format they were expecting so far.
Translated one test to stack format as example: reg-stackify-stack.ll

tested:
llvm-lit -v `find test -name WebAssembly`
unittests/MC/*

Reviewers: dschuff, sunfish

Subscribers: sbc100, jgravelle-google, eraman, aheejin, llvm-commits, jfb

Differential Revision: https://reviews.llvm.org/D51241

llvm-svn: 340750