[AMDGPU] Drop const from return types (NFC) (#145640)

We don't need const on these return types.

Author: kazutakahirata

llvm/lib/Target/AMDGPU/AMDGPURegBankLegalizeHelper.cpp

@@ -171,25 +171,22 @@ void RegBankLegalizeHelper::lowerVccExtToSel(MachineInstr &MI) {
   MI.eraseFromParent();
 }
 
-const std::pair<Register, Register>
-RegBankLegalizeHelper::unpackZExt(Register Reg) {
+std::pair<Register, Register> RegBankLegalizeHelper::unpackZExt(Register Reg) {
   auto PackedS32 = B.buildBitcast(SgprRB_S32, Reg);
   auto Mask = B.buildConstant(SgprRB_S32, 0x0000ffff);
   auto Lo = B.buildAnd(SgprRB_S32, PackedS32, Mask);
   auto Hi = B.buildLShr(SgprRB_S32, PackedS32, B.buildConstant(SgprRB_S32, 16));
   return {Lo.getReg(0), Hi.getReg(0)};
 }
 
-const std::pair<Register, Register>
-RegBankLegalizeHelper::unpackSExt(Register Reg) {
+std::pair<Register, Register> RegBankLegalizeHelper::unpackSExt(Register Reg) {
   auto PackedS32 = B.buildBitcast(SgprRB_S32, Reg);
   auto Lo = B.buildSExtInReg(SgprRB_S32, PackedS32, 16);
   auto Hi = B.buildAShr(SgprRB_S32, PackedS32, B.buildConstant(SgprRB_S32, 16));
   return {Lo.getReg(0), Hi.getReg(0)};
 }
 
-const std::pair<Register, Register>
-RegBankLegalizeHelper::unpackAExt(Register Reg) {
+std::pair<Register, Register> RegBankLegalizeHelper::unpackAExt(Register Reg) {
   auto PackedS32 = B.buildBitcast(SgprRB_S32, Reg);
   auto Lo = PackedS32;
   auto Hi = B.buildLShr(SgprRB_S32, PackedS32, B.buildConstant(SgprRB_S32, 16));

llvm/lib/Target/AMDGPU/AMDGPURegBankLegalizeHelper.h

@@ -111,9 +111,9 @@ class RegBankLegalizeHelper {
              SmallSet<Register, 4> &SgprWaterfallOperandRegs);
 
   void lowerVccExtToSel(MachineInstr &MI);
-  const std::pair<Register, Register> unpackZExt(Register Reg);
-  const std::pair<Register, Register> unpackSExt(Register Reg);
-  const std::pair<Register, Register> unpackAExt(Register Reg);
+  std::pair<Register, Register> unpackZExt(Register Reg);
+  std::pair<Register, Register> unpackSExt(Register Reg);
+  std::pair<Register, Register> unpackAExt(Register Reg);
   void lowerUnpackBitShift(MachineInstr &MI);
   void lowerV_BFE(MachineInstr &MI);
   void lowerS_BFE(MachineInstr &MI);