VarAction2: Remove operations which always do nothing when applied to 0

src/newgrf.cpp

@@ -5862,6 +5862,10 @@ static void OptimiseVarAction2Adjust(VarAction2OptimiseState &state, const GrfSp
 	} else if ((prev_inference & VA2AIF_HAVE_CONSTANT) && adjust.variable == 0x1A && IsEvalAdjustUsableForConstantPropagation(adjust.operation)) {
 		/* Reduce constant operation on previous constant */
 		replace_with_constant_load(EvaluateDeterministicSpriteGroupAdjust(group->size, adjust, nullptr, state.current_constant, UINT_MAX));
+	} else if ((prev_inference & VA2AIF_HAVE_CONSTANT) && state.current_constant == 0 && adjust.variable != 0x7E && IsEvalAdjustWithZeroLastValueAlwaysZero(adjust.operation)) {
+		/* Remove operation which does nothing when applied to 0 */
+		group->adjusts.pop_back();
+		state.inference = prev_inference;
 	} else if ((prev_inference & VA2AIF_HAVE_CONSTANT) && IsEvalAdjustOperationOnConstantEffectiveLoad(adjust.operation, state.current_constant)) {
 		/* Convert operation to a load */
 		DeterministicSpriteGroupAdjust current = group->adjusts.back();

src/newgrf_spritegroup.h

@@ -307,6 +307,27 @@ inline bool IsEvalAdjustOperationOnConstantEffectiveLoad(DeterministicSpriteGrou
 	}
 }
 
+inline bool IsEvalAdjustWithZeroLastValueAlwaysZero(DeterministicSpriteGroupAdjustOperation op)
+{
+	switch (op) {
+		case DSGA_OP_SDIV:
+		case DSGA_OP_SMOD:
+		case DSGA_OP_UDIV:
+		case DSGA_OP_UMOD:
+		case DSGA_OP_UMIN:
+		case DSGA_OP_MUL:
+		case DSGA_OP_AND:
+		case DSGA_OP_ROR:
+		case DSGA_OP_SHL:
+		case DSGA_OP_SHR:
+		case DSGA_OP_SAR:
+			return true;
+
+		default:
+			return false;
+	}
+}
+
 struct DeterministicSpriteGroupAdjust {
 	DeterministicSpriteGroupAdjustOperation operation;
 	DeterministicSpriteGroupAdjustType type;