[AMDGPU] Remove unused functions isSigned. NFC (#169750)

These have been unused since
llvm/llvm-project#145483.

Author: tclin914

llvm/lib/Target/AMDGPU/AMDGPUCodeGenPrepare.cpp

@@ -143,14 +143,6 @@ class AMDGPUCodeGenPrepareImpl
 
   bool canBreakPHINode(const PHINode &I);
 
-  /// \returns True if binary operation \p I is a signed binary operation, false
-  /// otherwise.
-  bool isSigned(const BinaryOperator &I) const;
-
-  /// \returns True if the condition of 'select' operation \p I comes from a
-  /// signed 'icmp' operation, false otherwise.
-  bool isSigned(const SelectInst &I) const;
-
   /// Return true if \p T is a legal scalar floating point type.
   bool isLegalFloatingTy(const Type *T) const;
 
@@ -304,16 +296,6 @@ bool AMDGPUCodeGenPrepareImpl::run() {
   return MadeChange;
 }
 
-bool AMDGPUCodeGenPrepareImpl::isSigned(const BinaryOperator &I) const {
-  return I.getOpcode() == Instruction::AShr ||
-      I.getOpcode() == Instruction::SDiv || I.getOpcode() == Instruction::SRem;
-}
-
-bool AMDGPUCodeGenPrepareImpl::isSigned(const SelectInst &I) const {
-  return isa<ICmpInst>(I.getOperand(0)) &&
-         cast<ICmpInst>(I.getOperand(0))->isSigned();
-}
-
 bool AMDGPUCodeGenPrepareImpl::isLegalFloatingTy(const Type *Ty) const {
   return Ty->isFloatTy() || Ty->isDoubleTy() ||
          (Ty->isHalfTy() && ST.has16BitInsts());