[WebAssembly] Change comments on SP writing back (NFC)

[aheejin]

Summary

We now write back not to memory but to __stack_pointer global.

lib/Target/WebAssembly/WebAssemblyFrameLowering.h

 #include "llvm/CodeGen/TargetFrameLowering.h"
 
 namespace llvm {
 class MachineFrameInfo;
 
 class WebAssemblyFrameLowering final : public TargetFrameLowering {
  public:
   /// Size of the red zone for the user stack (leaf functions can use this much
-  /// space below the stack pointer without writing it back to memory).
+  /// space below the stack pointer without writing it back to __stack_pointer
+  /// global).
   // TODO: (ABI) Revisit and decide how large it should be.
   static const size_t RedZoneSize = 128;
 
   WebAssemblyFrameLowering()
       : TargetFrameLowering(StackGrowsDown, /*StackAlignment=*/16,
                             /*LocalAreaOffset=*/0,
                             /*TransientStackAlignment=*/16,
                             /*StackRealignable=*/true) {}

lib/Target/WebAssembly/WebAssemblyFrameLowering.cpp

 /// variable, so it is loaded into a register in the prolog.
 bool WebAssemblyFrameLowering::needsSP(const MachineFunction &MF,
                                        const MachineFrameInfo &MFI) const {
   return MFI.getStackSize() || MFI.adjustsStack() || hasFP(MF) ||
          needsPrologForEH(MF);
 }
 
 /// Returns true if the local user-space stack pointer needs to be written back
-/// to memory by this function (this is not meaningful if needsSP is false). If
-/// false, the stack red zone can be used and only a local SP is needed.
+/// to __stack_pointer global by this function (this is not meaningful if
+/// needsSP is false). If false, the stack red zone can be used and only a local
+/// SP is needed.
 bool WebAssemblyFrameLowering::needsSPWriteback(
     const MachineFunction &MF, const MachineFrameInfo &MFI) const {
   assert(needsSP(MF, MFI));
   return MFI.getStackSize() > RedZoneSize || MFI.hasCalls() ||
          MF.getFunction().hasFnAttribute(Attribute::NoRedZone);
 }
 
 void WebAssemblyFrameLowering::writeSPToGlobal(