arch-vega: Add valid flag to ds_swizzle_b32

Currently the flag is just Load and there is a long comment explaining
why. This does not meet any of the scoreboard check requirements:

https://github.com/gem5/gem5/blob/develop/src/gpu-compute/scoreboard_check_stage.cc#L230-L241

Add a generic ALU flag as well so the instruction executes instead of
panicking.

Change-Id: I54b2d20d47fad5e8f05f927328433aab7db7d862

src/arch/amdgpu/vega/insts/ds.cc

@@ -1997,6 +1997,7 @@ namespace VegaISA
          * fits in better with the LDS pipeline logic.
          */
          setFlag(Load);
+         setFlag(ALU);
     } // Inst_DS__DS_SWIZZLE_B32
 
     Inst_DS__DS_SWIZZLE_B32::~Inst_DS__DS_SWIZZLE_B32()