[GISel] Return const APInt & from getIConstantFromReg. NFC (llvm#114320)

This matches what the call to ConstantInt::getValue() returns. Let the
caller make a copy if needed.

Author: topperc

llvm/include/llvm/CodeGen/GlobalISel/Utils.h

@@ -180,7 +180,7 @@ std::optional<int64_t> getIConstantVRegSExtVal(Register VReg,
                                                const MachineRegisterInfo &MRI);
 
 /// \p VReg is defined by a G_CONSTANT, return the corresponding value.
-APInt getIConstantFromReg(Register VReg, const MachineRegisterInfo &MRI);
+const APInt &getIConstantFromReg(Register VReg, const MachineRegisterInfo &MRI);
 
 /// Simple struct used to hold a constant integer value and a virtual
 /// register.

llvm/lib/CodeGen/GlobalISel/Utils.cpp

@@ -287,7 +287,8 @@ std::optional<APInt> llvm::getIConstantVRegVal(Register VReg,
   return ValAndVReg->Value;
 }
 
-APInt llvm::getIConstantFromReg(Register Reg, const MachineRegisterInfo &MRI) {
+const APInt &llvm::getIConstantFromReg(Register Reg,
+                                       const MachineRegisterInfo &MRI) {
   MachineInstr *Const = MRI.getVRegDef(Reg);
   assert((Const && Const->getOpcode() == TargetOpcode::G_CONSTANT) &&
          "expected a G_CONSTANT on Reg");