[AMDGPU] Fix warnings about unused variables [NFC]

Without this gcc warned like
 ../lib/Target/AMDGPU/GCNDPPCombine.cpp:371: warning: unused variable 'OpSelOpr' [-Wunused-variable]
   371 |       if (auto *OpSelOpr =
       |
 ../lib/Target/AMDGPU/GCNDPPCombine.cpp:388: warning: unused variable 'OpSelHiOpr' [-Wunused-variable]
   388 |       if (auto *OpSelHiOpr =
       |

Remove the unused variables.

Author: mikaelholmen

llvm/lib/Target/AMDGPU/GCNDPPCombine.cpp

@@ -368,8 +368,7 @@ MachineInstr *GCNDPPCombine::createDPPInst(MachineInstr &OrigMI,
       }
       // Validate OP_SEL has to be set to all 0 and OP_SEL_HI has to be set to
       // all 1.
-      if (auto *OpSelOpr =
-              TII->getNamedOperand(OrigMI, AMDGPU::OpName::op_sel)) {
+      if (TII->getNamedOperand(OrigMI, AMDGPU::OpName::op_sel)) {
         int64_t OpSel = 0;
         OpSel |= (Mod0 ? (!!(Mod0->getImm() & SISrcMods::OP_SEL_0) << 0) : 0);
         OpSel |= (Mod1 ? (!!(Mod1->getImm() & SISrcMods::OP_SEL_0) << 1) : 0);
@@ -385,8 +384,7 @@ MachineInstr *GCNDPPCombine::createDPPInst(MachineInstr &OrigMI,
         if (AMDGPU::hasNamedOperand(DPPOp, AMDGPU::OpName::op_sel))
           DPPInst.addImm(OpSel);
       }
-      if (auto *OpSelHiOpr =
-              TII->getNamedOperand(OrigMI, AMDGPU::OpName::op_sel_hi)) {
+      if (TII->getNamedOperand(OrigMI, AMDGPU::OpName::op_sel_hi)) {
         int64_t OpSelHi = 0;
         OpSelHi |= (Mod0 ? (!!(Mod0->getImm() & SISrcMods::OP_SEL_1) << 0) : 0);
         OpSelHi |= (Mod1 ? (!!(Mod1->getImm() & SISrcMods::OP_SEL_1) << 1) : 0);