[AMDGPU] Remove obsolete comment

Obsoleted by e4464bf "AMDGPU/GlobalISel: Select scalar v2s16 G_BUILD_VECTOR"

Author: jayfoad

llvm/lib/Target/AMDGPU/SIInstructions.td

@@ -2040,8 +2040,6 @@ def : GCNPat <
                   SRCMODS.NONE, $src2)
 >;
 
-// COPY is workaround tablegen bug from multiple outputs
-// from S_LSHL_B32's multiple outputs from implicit scc def.
 def : GCNPat <
   (v2i16 (build_vector (i16 0), (i16 SReg_32:$src1))),
   (S_LSHL_B32 SReg_32:$src1, (i16 16))