[GlobalISel]: Add a getConstantIntVRegVal utility

[Petar.Avramovic]

Summary

Returns ConstantInt from G_CONSTANT instruction given its def register.

[arsenm]

LGTM, although we have a few too many constant lookup functions at this point

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

 class MCInstrDesc;
 class ProfileSummaryInfo;
 class RegisterBankInfo;
 class TargetInstrInfo;
 class TargetLowering;
 class TargetPassConfig;
 class TargetRegisterInfo;
 class TargetRegisterClass;
+class ConstantInt;
 class ConstantFP;
 class APFloat;
 
 // Convenience macros for dealing with vector reduction opcodes.
 #define GISEL_VECREDUCE_CASES_ALL                                              \
   case TargetOpcode::G_VECREDUCE_SEQ_FADD:                                     \
   case TargetOpcode::G_VECREDUCE_SEQ_FMUL:                                     \
   case TargetOpcode::G_VECREDUCE_FADD:                                         \

 /// When \p HandleFConstants == false the function bails on G_FCONSTANTs.
 /// When \p LookThroughAnyExt == true the function treats G_ANYEXT same as
 /// G_SEXT.
 Optional<ValueAndVReg>
 getConstantVRegValWithLookThrough(Register VReg, const MachineRegisterInfo &MRI,
                                   bool LookThroughInstrs = true,
                                   bool HandleFConstants = true,
                                   bool LookThroughAnyExt = false);
+const ConstantInt *getConstantIntVRegVal(Register VReg,
+                                         const MachineRegisterInfo &MRI);
 const ConstantFP* getConstantFPVRegVal(Register VReg,
                                        const MachineRegisterInfo &MRI);
 
 /// See if Reg is defined by an single def instruction that is
 /// Opcode. Also try to do trivial folding if it's a COPY with
 /// same types. Returns null otherwise.
 MachineInstr *getOpcodeDef(unsigned Opcode, Register Reg,
                            const MachineRegisterInfo &MRI);

llvm/lib/CodeGen/GlobalISel/Utils.cpp

case TargetOpcode::G_ZEXT:
       Val = Val.zext(OpcodeAndSize.second);
       break;
     }
   }
 
   return ValueAndVReg{Val, VReg};
 }
 
+const ConstantInt *llvm::getConstantIntVRegVal(Register VReg,
+                                               const MachineRegisterInfo &MRI) {
+  MachineInstr *MI = MRI.getVRegDef(VReg);
+  if (MI->getOpcode() != TargetOpcode::G_CONSTANT)
+    return nullptr;
+  return MI->getOperand(1).getCImm();
+}
+
 const ConstantFP *
 llvm::getConstantFPVRegVal(Register VReg, const MachineRegisterInfo &MRI) {
   MachineInstr *MI = MRI.getVRegDef(VReg);
   if (TargetOpcode::G_FCONSTANT != MI->getOpcode())
     return nullptr;
   return MI->getOperand(1).getFPImm();
 }