[AMDGPU][NFC] Harmonize decl&def of R600TargetLowering::OptimizeSwizzle

The freshly baked -Warray-parameter warning discovered an inconsistency in
argument declaration, use the stricter one.

This fixes build issues like https://lab.llvm.org/buildbot#builders/18/builds/5305

Author: serge-sans-paille

llvm/lib/Target/AMDGPU/R600ISelLowering.h

@@ -74,8 +74,8 @@ class R600TargetLowering final : public AMDGPUTargetLowering {
 
   void lowerImplicitParameter(MachineInstr *MI, MachineBasicBlock &BB,
       MachineRegisterInfo & MRI, unsigned dword_offset) const;
-  SDValue OptimizeSwizzle(SDValue BuildVector, SDValue Swz[], SelectionDAG &DAG,
-                          const SDLoc &DL) const;
+  SDValue OptimizeSwizzle(SDValue BuildVector, SDValue Swz[4],
+                          SelectionDAG &DAG, const SDLoc &DL) const;
   SDValue vectorToVerticalVector(SelectionDAG &DAG, SDValue Vector) const;
 
   SDValue lowerFrameIndex(SDValue Op, SelectionDAG &DAG) const;