[WebAssembly][SelectionDAG] Remove unused WebAssemblyDAGToDAGISel::ForCodeSize.

[hjyamauchi]

Summary

This follows from the discussion at D70095.

D70095 moves hasOptSize calls into SelectionDAG::shouldOptForSize to allow
querying size optimization conditions together with profile guided size
optimization.

Since it appears that size optimizations for WebAssembly SelectionDAG haven't
been implemented yet and thus ForCodeSize is unused, and it would not make a lot
of sense to call shouldOptForSize here as the necessary profile data like
PSI/BFI aren't available at this point, it seems good and less confusing to
remove this for now and use shouldOptForSize when they are implemented in the
future.

[hjyamauchi]

Can someone suggest reviewers?

llvm/lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp

 /// SelectionDAG operations.
 ///
 namespace {
 class WebAssemblyDAGToDAGISel final : public SelectionDAGISel {
   /// Keep a pointer to the WebAssemblySubtarget around so that we can make the
   /// right decision when generating code for different targets.
   const WebAssemblySubtarget *Subtarget;
 
-  bool ForCodeSize;
-
 public:
   WebAssemblyDAGToDAGISel(WebAssemblyTargetMachine &TM,
                           CodeGenOpt::Level OptLevel)
-      : SelectionDAGISel(TM, OptLevel), Subtarget(nullptr), ForCodeSize(false) {
+      : SelectionDAGISel(TM, OptLevel), Subtarget(nullptr) {
   }
 
   StringRef getPassName() const override {
     return "WebAssembly Instruction Selection";
   }
 
   bool runOnMachineFunction(MachineFunction &MF) override {
     LLVM_DEBUG(dbgs() << "********** ISelDAGToDAG **********\n"
                          "********** Function: "
                       << MF.getName() << '\n');
 
-    ForCodeSize = MF.getFunction().hasOptSize();
     Subtarget = &MF.getSubtarget<WebAssemblySubtarget>();
 
     // Wasm64 is not fully supported right now (and is not specified)
     if (Subtarget->hasAddr64())
       report_fatal_error(
           "64-bit WebAssembly (wasm64) is not currently supported");
 
     return SelectionDAGISel::runOnMachineFunction(MF);