[GlobalISel] Fix warnings

This patch fixes:

  llvm/lib/CodeGen/GlobalISel/CombinerHelperCasts.cpp:167:21: error:
  unused variable 'DL' [-Werror,-Wunused-variable]

  llvm/lib/CodeGen/GlobalISel/LoadStoreOpt.cpp:320:15: error: unused
  variable 'DL' [-Werror,-Wunused-variable]

Author: kazutakahirata

llvm/lib/CodeGen/GlobalISel/CombinerHelperCasts.cpp

@@ -164,7 +164,6 @@ bool CombinerHelper::matchTruncateOfExt(const MachineInstr &Root,
 
 bool CombinerHelper::isCastFree(unsigned Opcode, LLT ToTy, LLT FromTy) const {
   const TargetLowering &TLI = getTargetLowering();
-  const DataLayout &DL = getDataLayout();
   LLVMContext &Ctx = getContext();
 
   switch (Opcode) {

llvm/lib/CodeGen/GlobalISel/LoadStoreOpt.cpp

@@ -317,7 +317,6 @@ bool LoadStoreOpt::mergeStores(SmallVectorImpl<GStore *> &StoresToMerge) {
     assert(MRI->getType(StoreMI->getValueReg()) == OrigTy);
 #endif
 
-  const auto &DL = MF->getFunction().getDataLayout();
   bool AnyMerged = false;
   do {
     unsigned NumPow2 = llvm::bit_floor(StoresToMerge.size());