[AMDGPU] Silence gcc warning [NFC]

Without the fix gcc complains with
 ../lib/Target/AMDGPU/SIWholeQuadMode.cpp:1543: warning: enumeral and non-enumeral type in conditional expression [-Wextra]
  1542 |     unsigned CopyOp = MI->getOperand(1).isReg()
       |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  1543 |                           ? AMDGPU::COPY
       |                           ~~~~~~~~~~~~~~
  1544 |                           : TII->getMovOpcode(TRI->getRegClassForOperandReg(
       |                           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  1545 |                                 *MRI, MI->getOperand(0)));
       |