[WebAssembly] Modified tablegen defs to have 2 parallel instuction sets.

Summary:
One for register based, much like the existing definitions,
and one for stack based (suffix _S).

This allows us to use registers in most of LLVM (which works better),
and stack based in MC (which results in a simpler and more readable
assembler / disassembler).

Tried to keep this change as small as possible while passing tests,
follow-up commit will:
- Add reg->stack conversion in MI.
- Fix asm/disasm in MC to be stack based.
- Fix emitter to be stack based.

tests passing:
llvm-lit -v `find test -name WebAssembly`

test/CodeGen/WebAssembly
test/MC/WebAssembly
test/MC/Disassembler/WebAssembly
test/DebugInfo/WebAssembly
test/CodeGen/MIR/WebAssembly
test/tools/llvm-objdump/WebAssembly

Reviewers: dschuff, sbc100, jgravelle-google, sunfish

Subscribers: aheejin, JDevlieghere, llvm-commits

Differential Revision: https://reviews.llvm.org/D48183

llvm-svn: 334985