[WebAssembly] Fix printing of global operands

[dschuff]

Summary

They still need to be printed as "i32.const" rather than "global".
(Fixed from r252656 which wasn't quite NFC)

[sunfish]

Actually, I believe there's a better way to fix this. X86 uses this, for example:

def : Pat<(i32 (X86Wrapper tglobaladdr :$dst)), (MOV32ri tglobaladdr :$dst)>;

We should be able to replace our GLOBAL instruction with essentially the same thing (our version of MOV32ri is just Const_I32). This will use i32.const instructions right from the start, so they should then go down the normal path in the assembly printer.

[dschuff]

• Use a pattern instead of a custom instruction

[jfb]

lgtm

lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp

assert(NumDefs <= 1 &&
          "Instructions with multiple result values not implemented");
 
   OS << '\t';
 
   switch (MI->getOpcode()) {
   case TargetOpcode::COPY:
     OS << "get_local push, " << regToString(MI->getOperand(1));
     break;
+  case WebAssembly::GLOBAL:
+    // TODO: wasm64
+    OS << "i32.const push, " << toSymbol(MI->getOperand(1).getGlobal()->getName());
+    break;
   case WebAssembly::ARGUMENT_I32:
   case WebAssembly::ARGUMENT_I64:
   case WebAssembly::ARGUMENT_F32:
   case WebAssembly::ARGUMENT_F64:
     OS << "get_local push, " << argToString(MI->getOperand(1));
     break;
   default: {
     OS << OpcodeName(TII, MI);

test/CodeGen/WebAssembly/global.ll

 ; RUN: llc < %s -asm-verbose=false | FileCheck %s
 
 ; Test that globals assemble as expected.
 
 target datalayout = "e-p:32:32-i64:64-n32:64-S128"
 target triple = "wasm32-unknown-unknown"
 
 ; CHECK-NOT: llvm.used
 ; CHECK-NOT: llvm.metadata
 @llvm.used = appending global [1 x i32*] [i32* @g], section "llvm.metadata"
 
+; CHECK: foo:
+; CHECK: i32.const push, $answer
+define i32 @foo() {
+  %a = load i32, i32* @answer
+  ret i32 %a
+}
+
 ; CHECK: .type   g,@object
 ; CHECK: .align  2{{$}}
 ; CHECK-NEXT: g:
 ; CHECK-NEXT: .int32 1337{{$}}
 ; CHECK-NEXT: .size g, 4{{$}}
 @g = private global i32 1337
 
 ; CHECK-LABEL: ud: