revert add cannonlization to sub in true16

Author: broxigarchen

llvm/lib/Target/AMDGPU/VOP3Instructions.td

@@ -1271,7 +1271,11 @@ let True16Predicate = NotHasTrue16BitInsts, SubtargetPredicate = isGFX10Plus in
 let True16Predicate = UseRealTrue16Insts in {
   def : OpSelBinOpClampPat<uaddsat, V_ADD_NC_U16_t16_e64>;
   def : OpSelBinOpClampPat<usubsat, V_SUB_NC_U16_t16_e64>;
-} // End OtherPredicates = [UseRealTrue16Insts]
+  def : GCNPat<
+    (add i16:$src0, (i16 NegSubInlineIntConst16:$src1)),
+    (V_SUB_NC_U16_t16_e64 0, VSrc_b16:$src0, 0, NegSubInlineIntConst16:$src1, 0, 0)
+  >;
+} // End True16Predicate = UseRealTrue16Insts
 
 let True16Predicate = UseFakeTrue16Insts in {
    def : OpSelBinOpClampPat<uaddsat, V_ADD_NC_U16_fake16_e64>;