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

Differential Revision: https://reviews.llvm.org/D50568

llvm-svn: 339474

llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp

@@ -317,9 +317,6 @@ void WebAssemblyPassConfig::addPreEmitPass() {
   // converted into a local.
   addPass(createWebAssemblyFixIrreducibleControlFlow());
 
-  // Insert explicit get_local and set_local operators.
-  addPass(createWebAssemblyExplicitLocals());
-
   // Do various transformations for exception handling
   addPass(createWebAssemblyLateEHPrepare());
 
@@ -337,6 +334,9 @@ void WebAssemblyPassConfig::addPreEmitPass() {
   if (getOptLevel() != CodeGenOpt::None)
     addPass(createWebAssemblyPeephole());
 
+  // Insert explicit get_local and set_local operators.
+  addPass(createWebAssemblyExplicitLocals());
+
   // Create a mapping from LLVM CodeGen virtual registers to wasm registers.
   addPass(createWebAssemblyRegNumbering());
 }