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

Summary:
Moved Explicit Locals pass to last.
Made that pass obligatory.
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

Differential Revision: https://reviews.llvm.org/D49160

llvm-svn: 338164
diff --git a/llvm/test/CodeGen/WebAssembly/function-bitcasts-varargs.ll b/llvm/test/CodeGen/WebAssembly/function-bitcasts-varargs.ll
index 5f1997b..56626b6 100644
--- a/llvm/test/CodeGen/WebAssembly/function-bitcasts-varargs.ll
+++ b/llvm/test/CodeGen/WebAssembly/function-bitcasts-varargs.ll
@@ -1,4 +1,4 @@
-; RUN: llc < %s -asm-verbose=false -wasm-temporary-workarounds=false | FileCheck %s
+; RUN: llc < %s -asm-verbose=false -wasm-temporary-workarounds=false -wasm-explicit-locals-codegen-test-mode | FileCheck %s
 
 ; Test that function pointer casts casting away varargs are replaced with
 ; wrappers.