[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/func.ll b/llvm/test/CodeGen/WebAssembly/func.ll
index cd78df0..17d2076 100644
--- a/llvm/test/CodeGen/WebAssembly/func.ll
+++ b/llvm/test/CodeGen/WebAssembly/func.ll
@@ -15,8 +15,8 @@
 ; CHECK-LABEL: f1:
 ; CHECK-NEXT: .result i32{{$}}
 ; CHECK-NEXT: .local i32{{$}}
-; CHECK-NEXT: i32.const push, 0{{$}}
-; CHECK-NEXT: set_local 0, pop{{$}}
+; CHECK-NEXT: i32.const $push, 0{{$}}
+; CHECK-NEXT: set_local 0, $pop{{$}}
 ; CHECK-NEXT: return (get_local 0){{$}}
 ; CHECK: .size f1,
 define i32 @f1() {
@@ -28,8 +28,8 @@
 ; CHECK-NEXT: .param f32{{$}}
 ; CHECK-NEXT: .result i32{{$}}
 ; CHECK-NEXT: .local i32{{$}}
-; CHECK-NEXT: i32.const push, 0{{$}}
-; CHECK-NEXT: set_local 2, pop{{$}}
+; CHECK-NEXT: i32.const $push, 0{{$}}
+; CHECK-NEXT: set_local 2, $pop{{$}}
 ; CHECK-NEXT: return (get_local 2){{$}}
 ; CHECK: .size f2,
 define i32 @f2(i32 %p1, float %p2) {