[WebAssembly] Switch to MC for instruction printing.

This encompasses several changes which are all interconnected:
 - Use the MC framework for printing almost all instructions.
 - AsmStrings are now live.
 - This introduces an indirection between LLVM vregs and WebAssembly registers,
   and a new pass, WebAssemblyRegNumbering, for computing a basic the mapping.
   This addresses some basic issues with argument registers and unused registers.
 - The way ARGUMENT instructions are handled no longer generates redundant
   get_local+set_local for every argument.

This also changes the assembly syntax somewhat; most notably, MC's printing
use sigils on label names, so those are no longer present, and push/pop now
have a sigil to keep them unambiguous.

The usage of set_local/get_local/$push/$pop will continue to evolve
significantly. This patch is just one step of a larger change.

llvm-svn: 252858
diff --git a/llvm/test/CodeGen/WebAssembly/memory-addr32.ll b/llvm/test/CodeGen/WebAssembly/memory-addr32.ll
index 194fd64..9193d79 100644
--- a/llvm/test/CodeGen/WebAssembly/memory-addr32.ll
+++ b/llvm/test/CodeGen/WebAssembly/memory-addr32.ll
@@ -12,7 +12,7 @@
 ; CHECK-NEXT: .result i32{{$}}
 ; CHECK-NEXT: .local i32{{$}}
 ; CHECK-NEXT: memory_size
-; CHECK-NEXT: set_local 0, pop{{$}}
+; CHECK-NEXT: set_local 0, $pop{{$}}
 ; CHECK-NEXT: return (get_local 0){{$}}
 define i32 @memory_size() {
   %a = call i32 @llvm.wasm.memory.size.i32()
@@ -22,7 +22,7 @@
 ; CHECK-LABEL: grow_memory:
 ; CHECK-NEXT: .param i32
 ; CHECK-NEXT: .local i32{{$}}
-; CHECK: grow_memory (get_local 1)
+; CHECK: grow_memory (get_local 0)
 ; CHECK-NEXT: return
 define void @grow_memory(i32 %n) {
   call void @llvm.wasm.grow.memory.i32(i32 %n)